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4T2. Description of the left training handle assembly. The left training handle assembly operates the prism tilt mechanism by the movement of the revolving grip, and is interconnected with an

It is equipped with a spring detent to hold the line of sight at elevations of 14 degrees and 44 degrees above horizontal. The detent facilitates observation of the entire sky. This is done by placing the periscope in low power and observing in three zones with the line of sight set respectively at 1) 14 degrees elevation; 2) 44 degrees elevation; and 3) full, or 74.5 degrees, elevation. If the periscope is rotated a full revolution in azimuth in each position, the entire sky is seen with a minimum of overlap between the zones. The detent may be rendered inoperative by rotating the plunger release knob (35, Figure 4-43). Figure 4-43 shows the left training handle assembly. All bubble numbers in Sections 4J2, 3, and 4 refer to Figure 4-43 unless otherwise specified.

The outer counterbored end carries an outer collar (3), a press fit in the counterbored section, and an end cap (1) which is also a press fit in the outer part of this same counterbored section. A reamed clearance hole in the revolving grip and the outer collar (3) is provided for the lockscrew (12). The inner counterbored end carries an inner collar (5), a press fit in this counterbored section.

b. Revolving grip outer collar. The revolving grip outer collar (3) is made of composition brass and is cylindrical. The periphery is a press fit in the outer counterbored section end in the revolving grip (2). It has a reamed hole in the center axis, a sliding fit on the revolving grip shaft (30). A reamed hole in this collar coinciding with the reamed hole in the revolving grip wall accommodates a lockscrew (12) which screws into the tapped hole in the revolving grip shaft (30). This lockscrew secures the revolving grip and the other collar to the shaft for its operation

c. Revolving grip end cap. The revolving grip end cap (1) is made of brass rod and is cylindrical. The large narrow shoulder flange diameter coincides with the diameter of the outer end radius shoulder of the revolving grip (2) when the undercut shoulder is pressed into the outer end counterbored section in the revolving grip. The outer sharp corner of the large narrow shoulder flange is rounded off. This end cap covers the outer part of the revolving grip, thus preventing the entry of foreign matter. The small drilled hole in the center axis serves as an air release hole as the revolving grip is assembled on the revolving grip shaft (30).

d. Revolving grip inner collar. The revolving grip inner collar (5) is made of composition brass and is cylindrical. The periphery is a press fit in the inner end counterbored section of the revolving grip (2). It has a reamed hole and counterbored section. The reamed hole is a

e. Fixed grip. The fixed grip (29) is made of brass tubing and is 3 3/4 inches in length. The periphery is rough diamond knurled to offer the observer a firm grip. Both ends of the knurled periphery are relieved. The inner end has a small radius, while the outer end is provided with the same type of radius except that it has an undercut shoulder 7/32 inch in length. This undercut shoulder diameter conforms to the diameter of the index ring (6) and has the stationary index line engraved in its shoulder at assembly.

The counterbored section in the outer end carries the large shoulder section of the outer collar (4), a press fit in this counterbored section. The inner end is counterbored a depth of 1 1/2 inches, and is a sliding fit on the alignment support section of the handy hinge (28). It is secured on this alignment support section with a lockscrew (42), which extends from the tapped hole in the fixed grip and further into the tapped hole in the alignment support section of the handle hinge (28).

Directly opposite this tapped hole, a large tapped hole is located outward with an approximate variance of 9/16-inch counter distance. This tapped hole carries the detent plunger housing (34).

f. Fixed grip outer collar. The fixed grip outer collar. (4) is made of composition brass and is 1 3/8 inches in length. It is cylindrical, and has a reamed hole in its center axis, a sliding fit on the revolving grip shaft (30). The large shoulder section is a press fit in the fixed grip (29). The undercut shoulder section projects outward from the outer end of the fixed grip, and is provided with a diameter of 1 1/2

The remaining undercut shoulder section extends into the counterbored section in the revolving grip inner collar (5) The outer part of the undercut shoulder has a cutaway semicircular section. The remaining semicircular section serves as a segment stop foundation. This section is provided with two tapped holes on opposite sides perpendicular to the split and in the center of the split wall thickness, for the insertion of two segment stop adjusting screws (26). The projecting part of each tapped hole in the remaining periphery is recessed to provide clearance for the insertion of a screwdriver blade. The adjusting screws, project into the milled-out part of the semicircular section to contact the segment stop (7) attached to the revolving grip shaft (30) for adjustment of the index ring (6).

The side face of the semicircular projection section is provided with opposite tapped holes in the centerline and at a perpendicular plane to the adjusting screws (26) for two segment stop adjusting screw lockscrews (25). After adjustments have been made with the adjusting screws (26), they are secured with these two lockscrews (25) to maintain the adjustments.

g. Revolving grip shaft. The revolving grip shaft (30) is made of brass tubing and is 8.375 inches in length. This shaft is a sliding fit in the reamed hole in the revolving rip outer collar, (3) and the inner collar (5). It has a tapped hole near its outer part to receive the threaded part of the lockscrew (12) which projects inward from the clearance hole in the revolving grip (2) and its outer collar (3) for the manipulation of the shaft upon the rotation of the revolving grip (2).

Two tapped holes are provided in the shaft for the segment stop lockscrews (23) at assembly to secure the segment stop (7) for its proper location in the counterbored section in the revolving grip inner collar (5).

The inner part of the shaft is a sliding fit in the reamed hole in the fixed grip outer collar (4) and extends the entire length of the fixed grip (29), a sliding fit in the large reamed hole in the handle hinge (28). The inner end of the

h. Index ring. The index ring (6) is made of composition brass and is cylindrical, with a width of 5/16 inch. The bored hole is a sliding fit on the undercut shoulder of the fixed grip outer collar (4). The periphery is engraved after assembly to indicate 10 degrees depression, 0 degrees, 14 degrees, 44 degrees, and 74.5 degrees elevation. The side face has a drilled hole with a radially sawed slot to permit adjustment so that the index ring fits snugly on the outer collar, providing sufficient friction so that it does not slide free when elevating or depressing the head prism by the rotation of the revolving grip (2). A circumferential recess slot 0.375 inch in length is opposite the drilled hole. The screw head of the index ring actuating screw (22) projecting from the side face of the revolving grip inner collar (5) engages in this recess slot. This recess slot has 3/32-inch movement to coordinate with the correction made with the adjusting screws (26) in the fixed grip outer collar semicircular section (4). The index ring fits between the inner face of the revolving grip (2) and the outer face of the fixed grip (29). The graduations are read when they coincide with the stationary engraved reference line on the fixed grip.

i. Segment stop. The segment stop. (7) is made of composition brass. It consists of a segment of approximately 60 degrees, with an inside radius of 3/8 inch and an external radius of 9/16 inch. The outside radius conforms to the contour of the undercut shoulder periphery of the fixed grip outer collar (4), while the inside radius conforms to the contour of the revolving grip shaft periphery (30). It is secured to the revolving grip shaft with two lockscrews (23). These lockscrews are inserted into countersunk clearance holes in the segment stop (7) and screwed into tapped holes in the revolving grip shaft (30) located in the counterbored section of the revolving grip inner collar (5). The segment stop is rotated with the revolving grip shaft (30) and contacts the adjusting screws (26) for full depression and elevation, plus

j. Handle hinge. The handle hinge (28) is made of cast phosphor bronze and is approximately 5 inches in length. It forms the outer moving hinge part of the training handle assembly. The outer part has a turned alignment support section 1 1/2 inches long, with a narrow shoulder following this section. This alignment support section serves as a stabilizing support for the inner counterbored section of the fixed grip (29), which is a snug sliding fit on this alignment support section, and is secured with a lockscrew (42). This lockscrew screws into a tapped hole in the fixed grip (29) and extends into the tapped hole in the alignment support section wall.

The filleted cast section between the alignment support section shoulder and the hinge section wall forms a cylindrical extension between these sections. It is provided with a raised boss in the upper rear part to provide the necessary wall thickness for the retention of a handle detent plunger assembly.

The hinge section is similar in shape to an apron, with the contour of the outer circumference uniform with the inner wall circumference with a radius of approximately 140 degrees. The side walls of the hinge section have projecting bosses on the inner and outer faces, with a reamed hole through the center axis of each boss offset from the main horizontal centerline. The inner bosses are a sliding fit over the side walls of the stationary hinge section of the hinge bracket (27). The reamed holes in each side wall of the movable hinge section carry a pivot screw (20), thus serving as hinge pivots to carry the handle hinge (28) through 90 degrees rotation.

The inner circumference of the apron wall of the hinge section has sufficient clearance over the stationary hinge section wall periphery of the hinge bracket (27) to allow unrestricted movement for the folding and unfolding of the handle hinge (28). In the extended or unfolded position, the lower flat face of the apron section rests against the upper rectangular center face of the hinge bracket (27).

The inner surface of the handle hinge is provided with two counterbored sections in

The large counterbored section wall is provided with a square broached hole for the square section of the detent plunger (33) and an opposite large clearance hole used for the broaching of this square hole. A shallow tapped hole in the same centerline and near the clearance hole receives the lockscrew (42).

The large reamed hole serves as an alignment support section for the inner end of the revolving grip shaft (30) of a sliding fit. The small reamed hole extends through the inner circumference of the cast apron wall. This small reamed hole carries the stem section of the outer bevel gear clutch shaft (8) secured to the revolving grip shaft (30). The small reamed hole is counterbored sufficiently in the inner circumference wall of the apron section to allow a partially flat surface for the assembly and the bearing contact of the outer bevel gear clutch collar (9).

A reamed hole extends outward from the inner circumference wall of the apron section, into the raised boss provision of the cast filleted section, a distance of 1 1/2 inches. This reamed hole carries a handle detent plunger spring (17) and a handle detent plunger (16). The plunger is a sliding fit in this seamed hole, and is secured by a lockscrew (18). This lockscrew extends inward from the tapped hole in the rear hinge section side wall for its protrusion into A the axial recess keyway in the handle detent plunger (16). The handle detent plunger rides on the rear stationary hinge section side Wall periphery of the hinge bracket (27) under spring tension and engages in a 90 degrees V-groove notch to retain the movable handle hinge (28) in the folded or vertical position.

A small clearance hole is provided in the centerline of the lower part of the apron wall to allow sufficient clearance for the removal of the outer bevel gear clutch shaft and collar taper pin (24).

k. Main body stop. The main body stop (31) is made of bronze and is 1.750 inches in

The outer part of the large shoulder section has a semicircular section 3/8 inch wide removed in the same manner as the fixed grip outer collar (4). It is also provided with two adjusting screws (26) and two adjusting screw lockscrews (25) in the same manner for this remaining semicircular section. The adjusting screws project into the milled-out semicircular part so that the detent 90 degrees V-groove notches of 14 degrees and 44 degrees elevation are synchronized with the 3/32-inch lost motion of the revolving grip (2).

The 90 degrees V-groove notches are so located in the large shoulder that they provide an indication by means of the 90 degrees formed detent plunger (33) under tension of a spring for the observer to determine the location of the 14 degrees and 44 degrees positions when observing the zenith.

1. Main body stop segment. The main body stop segment (32) is made of bronze. It consists of a segment 3/8 inch wide and approximately 165 degrees, with an inside radius of 3/8 inch and an external radius of 39/64 inch. The inside radius conforms to the contour of the revolving grip shaft periphery (30), while the outside radius is larger than the contour of the large shoulder of the main body stop periphery (31). The segment is secured to the revolving grip shaft with two lockscrews (39). These lockscrews are inserted in countersunk clearance holes in the main body stop segment (32) and screwed into tapped hole in the revolving grip shaft (30) located in coincidence with the main body stop semicircular protruding section (31). The main body stop is rotated with the revolving grip shaft and contacts the adjusting screws (26) for the rotation of the main body stop (31) for its use with the projecting detent plunger (33) located in the square broached hole in the handle hinge alignment support section (28).

m. Hinge bracket. The hinge bracket (27) is made of cast phosphor bronze, with a rectangular base. The hinge section projects

The inner face of the rectangular base is provided with a counterbored section and a reamed hole, offset from the horizontal centerline. The reamed hole serves as a bearing for the inner bevel gear clutch (14), while the counterbored section provides clearance over the left training handle packing gland assembly protruding stuffing box body flange (5, Figure 4-34) located in the eyepiece box. Two countersunk clearance holes and a tapped section are provided in the face of the counterbored section. These holes extend outward into both of the hinge section side walls and their perpendicular tapped holes for two pivot screw lockscrews (21). These lockscrews secure the pivot screws (20) when assembled in the hinge section side walls.

The central part of the hinge section is provided with a cylindrical raised boss, to carry the shoulder of the inner bevel gear clutch (14). Sufficient radius clearance is provided for assembly and removal of the inner and outer bevel gear clutches (14 and 15) and clearance inside the side walls for the 90 degrees rotation of the outer bevel gear clutch collar (9). The contour of the outer circumference of the side walls and the lower wall conforms to the inner circumference of the hinge section wall of the handle hinge (28). The rear side wall is carried above the upper flat wall approximately 1 1/16 inches and is provided with a 90 degrees V-groove notch in the same vertical centerline as the pivot screw tapped hole. The 90 degrees V-groove notch serves to retain the handle hinge (28) in the folded position by means of the handle detent plunger (16) under spring tension, and allows the handle hinge to swing downward of its own gravity by the force required to overcome the spring pressure of the handle detent plunger spring (17). The handle detent plunger rides on the periphery of the rear side wall, as the handle hinge is swung to the extended position. The

The inner face of each raised boss of the handle hinge (28) side walls is a sliding fit over the hinge section side walls of the hinge bracket (27). The pivot screws (20) extending from the opposite reamed holes in the handle hinge (28) extend into the tapped holes in the hinge section side walls of the hinge bracket (27) with the medium shoulder face of each pivot screw a metal to metal fit with the hinge section side walls.

n. Pivot screws. The pivot screws (20) are made of phosphor bronze and are 0.906 inch in length, with-the head section chromium plated. They form hinge pins on which the hinge section of the handle hinge (28) can be swung through 90 degrees rotation. Each screw has a slotted head section for a screwdriver blade. The head section projects outward from each side wall raised boss of the handle hinge (28). The main body section is a snug fit in reamed pivot holes in the hinge section side walls of the handle hinge, with this shoulder resting against the side wall faces of the hinge bracket hinge section (27). The stub section is threaded and engages in a tapped hole in each hinge section side wall of the hinge bracket (27). The pivot screws are secured with lockscrews (21) which are inserted into countersunk clearance holes in the counterbored section base of the hinge bracket (27) and screwed into tapped holes in each hinge section side wall to contact the threaded stub section of the pivot screws.

o. Outer bevel gear clutch shaft. The outer bevel gear clutch shaft (8) is made of monel metal and is 3 inches in length. The large diameter section is a pressed fit into the inner counterbored section end of the revolving grip shaft (30) and is secured with a taper pin (13). The stem section is a sliding fit into the small reamed hole in the handle hinge (28), and receives an outer bevel gear clutch collar (9) at the opposite end and the inner circumference end of the apron wall and hinge section. The outer bevel gear clutch collar (9) is secured to the stem section of the shaft with a taper pin (24) in the hinge section of the handle- hinge. The square section of the shaft carries the outer bevel gear clutch (15) against the spring tension

p. Outer bevel gear clutch collar. The outer bevel gear clutch collar (9) is made of phosphor bronze and is 0.656 inch in length. It provides a container in which the outer bevel gear clutch spring (10) is carried. It has a reamed hole in its center axis with a counterbored section, and is secured to the stem section of the outer bevel gear clutch shaft (8) with a taper pin (24). The outer bevel gear clutch spring (10) is carried over part of the stem section and the square section of the outer bevel gear clutch shaft (8). The spring places a constant tension against the hub face of the outer bevel gear clutch (15).

q. Inner and outer bevel gear clutches. The inner and outer bevel gear clutches (14 and 15) are made of phosphor bronze and are chromium plated. Both the bevel gear sections have the same diameter and number of teeth. They are provided with 19 bevel teeth of 20 diametral pitch, and have a pitch cone line angle of 45 degrees. Each is provided with a square broached hole. The square broached hole and the hub sections of the outer bevel gear clutch (15) move axially in the outer bevel gear clutch collar (9) against the outer bevel gear clutch spring (10) on the square section of the outer bevel gear clutch shaft (8).

The hub section of the inner bevel gear clutch fits in the reamed hole axis of the hinge bracket (27), and it extends farther on the square section of the actuating shaft (11, Figure 4-36) of the training handle packing gland assembly. It extends simultaneously on the square section of the shaft and in the counterbored recess in the packing gland (8).

The inner and outer bevel gear clutches are in mesh in either the folded or extended positions by means of the outer bevel gear clutch spring (10). In the folded position, both bevel gears are in perpendicular relation to each other at 90 degrees, with both 45 degrees pitch cone line angles. In the extended position, both level gears act as a universal jaw clutch, with all teeth engaged for the operation of the prism tilt mechanism.

The small shoulder serves to center the detent plunger spring concentrically, while the stem shaft extends through the reamed hole in the detent plunger spring retaining bushing (36), detent plunger release knob (3S), and the detent plunger retaining cap (37).

s. Detent plunger housing. The detent plunger housing (34) is made of brass rod 5/8 inch in length and chromium plated. The center axis is provided with a reamed hole to carry the large shoulder of the detent plunger (33) axially and has sufficient space for the detent plunger spring (38). The outer end has a threaded counterbored section of shallow depth to receive the threaded periphery shoulder of the detent plunger spring retaining bushing (36).

The inner end periphery is threaded a short distance, screws into the large tapped hole in the fixed rip (29), and rests against the flat spot face in the handle hinge alignment support section periphery (28). The outer face is provided with two opposite slots for a special wrench. A tapped hole in the wall periphery accommodates a detent plunger retaining bushing lockscrew (40), the head of which projects from the wall periphery the thickness of the detent plunger release knob undercut shoulder (35). This projecting lockscrew head offers the detent plunger release knob a contact support for the engagement or disengagement of the detent plunger (33) by turning the knob, thus allowing it to raise or lower the detent plunger.

t. Detent plunger spring retaining bushing. The detent plunger spring retaining bushing (36) is made of phosphor bronze and is 0.445 inch in length. It is provided with a large

u. Detent plunger spring. The detent plunger spring (38) is made of spring tempered phosphor-bronze wire having a free length of 0.870 inch and a coiled diameter of 0.280 inch. The spring is compressed in the detent plunger housing (34) by the detent plunger spring retaining bushing (36) and forces the detent plunger (33) into the 90 degrees V-groove notches in the main body stop large shoulder periphery in the engaged position, for 14 degrees and 44 degrees line of sight of the head prism (55, Figure 4-17).

v. Detent plunger release knob. The detent plunger release knob (35) is made of brass rod 1/2 inch in length and, chromium plated. The large shoulder periphery is knurled, having the sharp corner rounded off. The center axis has a reamed hole for the stem section of the detent plunger (33). It is provided with a counterbored section, a sliding fit on the detent plunger housing (34), leaving a nominal outer side wall. The undercut shoulder side face is provided with a shallow notch which rides in spring contact with the lockscrew head (40). When the shallow notch is in contact with the lockscrew head, the detent plunger is engaged for operation. The rotation of the knob causes the disengagement of the detent plunger (33).

w. Detent plunger retaining cap. The detent plunger retaining cap (37) is made of corrosion-resisting steel material. A reamed hole of shallow depth is located in its center axis, a sliding fit on the upper part of the detent plunger stem section and is secured with a lockscrew (41). This lockscrew is screwed into

x. Handle detent plunger spring. The handle detent plunger spring (17) is made of spring steel and has a free length of 1 3/4 inches. The spring is coiled to a diameter of 9/32 inch, and is a loose fit in the reamed hole of 1-inch depth in the handle detent plunger (16). The spring maintains a constant tension against the handle detent plunger (16) which is engaged in the 90 degrees V-groove notch in the rear side wall periphery of the hinge bracket hinge section (27) in the folded position. In the extended position, the spring is under full compression.

y. Handle detent plunger. The handle detent plunger (16) is made of corrosion resisting steel and is 1.593 inches in length. The outer end is provided with a reamed hole 1 inch deep, serving as a guide for the handle detent plunger spring (17). The inner end of the plunger is provided with a 90 degrees V-formed detent point for engagement in the 90 degrees V-groove notch in the hinge bracket hinge section side wall periphery (27) in the folded position.

The external diameter is a sliding fit in the reamed hole in the rear raised boss section, between the hinge section and the alignment support section, and a part of the filleted circular section of the handle hinge. The plunger, under heavy tension, projects outward from the apron wall of the handle hinge (28). A shallow keyway is provided at a perpendicular plane to the 90 degrees V-formed detent point. This keyway receives the undercut shoulder of the retaining screw (18) which extends inward from the tapped hole in the rear side wall face of the handle hinge (28). The undercut part of the retaining screw (18) engaged in the keyway prevents loss and injury upon disassembly of the handle hinge (28) in case the handle detent plunger is improperly secured.

The handle detent plunger (16) and spring (17) serve as a friction catch to retain the handle hinge in the folded position, by the engagement

4T3. Disassembly of the left training handle assembly. The left training handle assembly is disassembled in the following manner:

1. Remove they detent plunger assembly from the fixed grip (29), unscrewing the detent plunger housing (34) from the large tapped hole in the fixed grip.

2. Remove the lockscrew (41), unscrewing it from the detent plunger retaining cap (37). Remove the retaining cap.

3. Remove the detent plunger release knob (35), the detent plunger (33), and the detent plunger spring (38) from the inner end of the detent plunger housing (34).

4. Remove the detent plunger spring retaining bushing (36), using a special wrench inserted in the opposite holes to unscrew it from the detent plunger housing (34).

5. Remove the lockscrew (40), unscrewing it from the detent plunger housing (34).

6. Remove the lockscrew (12), unscrewing it from the tapped hole in the revolving grip shaft (30), and carrying it out of the revolving grip (2) and outer collar clearance holes (3).

7. Slide the revolving grip (2) off the revolving grip shaft (30), carrying with it the revolving grip end cap (1), outer collar (3), inner collar (5), and index ring actuating screw (22).

8. Remove the two lockscrews (23) from the segment stop (7), unscrewing them from tapped holes in the revolving rip shaft (30). Remove the segment stop (7).

9. Remove the lockscrew (42) from the fixed grip (29), unscrewing it from the tapped holes in the handle hinge alignment support section (28) and the fixed grip.

10. Remove the fixed grip (29 with the index ring (6) on the fixed grip outer collar (4), sliding it off the handle hinge alignment support section (28), and carrying it off the, revolving grip shaft (30).

12. Remove the two lockscrews (39) from the main body stop segment (32), unscrewing these lockscrews from tapped holes in the revolving grip shaft (30). Remove the main body stop segment (32).

13. Remove the main body stop (31) sliding it off the revolving grip shaft (30).

14. Remove the two pivot screw lockscrews (21), unscrewing them from contact with the two pivot screws (20) and the tapped holes in each hinge section side wall of the hinge bracket (27) in its inner counterbored recess in the base.

15. Swing the handle hinge to the extended position. Only in this position is there sufficient clearance for the removal of the outer bevel gear clutch (15) with the remaining assembly of the handle hinge (28) from the hinge bracket (27).

16. Remove the two pivot screws (20), unscrewing them from the tapped holes in the hinge section side walls of the hinge bracket (27). Remove the handle hinge assembly from the hinge bracket (27).

17. Remove the inner bevel gear clutch (14), sliding it out of the hinge bracket (27).

18. Remove the retaining screw (11), unscrewing it from the tapped hole in the outer bevel gear clutch shaft (8). Remove the outer bevel gear clutch (15) and the outer bevel gear clutch spring (10), sliding them off the square section of the outer bevel gear clutch shaft (8).

19. Rotate the revolving rip shaft (30) until the small end of the taper pin (24) is lined up with the drift clearance hole in the handle hinge wall (28).

20. Place a drift punch of suitable size in the handle hinge (28) clearance hole.

21. Drive the taper pin (24) from the outer bevel gear clutch collar (9) and the outer bevel gear clutch shaft (8).

22. Remove the outer bevel gear clutch collar (9) from the outer bevel gear clutch shaft (8).

24. Do not disassemble the outer bevel gear clutch shaft (8) from the revolving grip shaft (30). Leave them secured with the taper pin (13).

25. Remove the retaining screw (18), unscrewing it from its engagement in the keyway in the handle detent plunger (16), and the tapped hole in the hinge section rear side wall of the handle hinge (28).

26. Remove the handle detent plunger (16) and the handle detent plunger spring (17) from the reamed hole in the hinge section inner circumference wall of the handle hinge (28).

27. The two main body stops, the two segment stop adjusting screws (26), and the four lockscrews (25) are not altered during disassembly.

4T4. Reassembly of the left training, handle assembly. The left training handle assembly is reassembled in the following manner:

1. Lubricate lightly all rotating parts with Lubriplate No. 110 as the reassembly procedure is followed.

2. Place the handle detent plunger spring (17) in the handle detent plunger (16).

3. Place the handle detent plunger (16) and its spring (17) in the reamed hole in the rear inner circumference of the handle hinge (28). Rotate the handle detent plunger until the keyway is located to the rear, and its detent point is lying in a horizontal plane so that the, retaining screw (18) engages in the keyway.

4. Insert the retaining screw (18), screwing it into the tapped hole so that its undercut shoulder engages into the keyway in the handle detent plunger (16).

5. Place the assembled outer bevel gear clutch shaft (8) and revolving grip shaft (30) in their respective teamed holes in the handle hinge (28).

6. Place the outer bevel gear clutch collar (9) on the outer bevel gear clutch shaft (8).

7. Align the taper pin holes in the outer bevel gear clutch shaft (8) and collar (9).

9. Place the outer bevel gear clutch spring (10) over the outer bevel gear clutch shaft (8) and in the counterbored section of the outer bevel gear clutch collar (9).

10. Place the outer bevel gear clutch (15) on the square section of the outer bevel gear clutch shaft (8) with the reference marks in line.

11. Compress the outer bevel gear clutch spring (10) by pressing inward on the outer bevel gear clutch (15) for the insertion of the retaining screw (11). Insert the retaining screw (11), screwing it into the square section axis tapped hole in the outer bevel gear clutch shaft (8).

12. Check the outer bevel gear clutch (15) for free spring movement.

13. Place the inner bevel gear clutch (14) in the reamed hole in the cored hinge section of the hinge bracket (27).

14. Holding the handle hinge assembly in the extended position, carry the outer bevel gear clutch (15) through the cored clearance section in the hinge bracket (27).

15. Check the reference marks of the inner bevel gear clutch (14) tooth with its mating reference mark in the outer bevel gear clutch (15). Engage the gear teeth of the inner and outer bevel gear clutches, carrying the hinge section of the handle hinge (28) over the hinge section of the hinge bracket (27).

16. Apply downward pressure to the handle hinge (28); the handle detent plunger (16) resting on the hinge section side wall periphery of the hinge bracket (27) compresses the spring fully for the insertion of the two opposite side pivot screws (20).

17. Insert the two pivot screws (20) in the opposite side walls of the handle hinge (28), check the reference marks, and screw them into tapped holes in the hinge section side walls of the hinge bracket (27).

18. Secure both pivot screws (20) with the lockscrews (21), insert them in body clearance

19. Place the main body stop (31) on the revolving grip shaft (30), sliding it into the small and large counterbored sections in the alignment support section of the handle hinge (28).

20. Place the main body stop segment (32) on the revolving grip shaft (30); secure it opposite the semicircular projecting section of the main body stop (31) to the revolving grip shaft (30) with two lockscrews (39). These lockscrews are inserted in countersunk clearance holes in the main body stop segment (32) and screwed into tapped holes in the shaft.

21. Place the fixed grip (29) on the revolving grip shaft (30), sliding it on the alignment support section of the handle hinge (28).

22. Align the tapped lockscrew holes and insert the lockscrew (42). This lockscrew screws the tapped hole in the fixed grip (29) and the handle hinge alignment support section wall (28).

23. Place the index ring (6) over the revolving grip shaft (30) and on the undercut shoulder section of the fixed grip outer collar (4). It should fit snugly on the shoulder of this collar.

24. Place the segment stop (7) on the revolving grill shaft (30). Secure it opposite the semicircular projecting section of the fixed grip outer collar (4) to the revolving grip shaft (30) with two lockscrews (23). These lockscrews are inserted in countersunk clearance holes in the segment stop (7) and screwed into tapped holes in the shaft.

25. Place the revolving grip (2) on the revolving grip shaft (30), carrying with it the outer and inner collars (3 and 5), the end cap (1), and the index ring actuating screw (22). Engage the actuating screw head in the elongated radial recess ins the outer face of the index ring (6).

26. Insert the lockscrew (12), carrying it in the clearance holes of the revolving grip (2) and the outer collar (3), and screwing it into the tapped hole in the revolving grip shaft (30).

28. Insert the detent plunger release knob lockscrew (40) in the tapped hole in the detent plunger housing (34).

29. Insert the detent plunger spring retaining bushing (36), screwing it in the threaded counterbored section in the detent plunger housing (34), using a special wrench inserted in the opposite holes in its large shoulder face.

30. Place the detent plunger spring (38) in the detent plunger housing (34) from the inner end.

31. Place the detent plunger (33) in the detent plunger spring (38), detent plunger housing (34), and in the reamed hole in the detent plunger spring retaining bushing (36).

32. Place the lockscrew (41) in the tapped hole in the detent plunger retaining cap (37).

33. Place the detent plunger release knob (35) and the detent plunger retaining cap (37) on the protruding stem of the detent plunger (33). Holding the detent plunger release knob (35) and the end of the detent plunger (33), compress the detent plunger spring (38), carrying the stem section of the detent plunger outward and securing it with the lockscrew (41). The lockscrew contact a spotted face in the detent plunger stem section.

34. Place the detent plunger assembly in the fixed grip (29). The reference punch mark on the square part of the detent plunger should face upward. Insert the square part of the detent plunger (33) in the square broached hole in the handle hinge (28). Screw the detent plunger housing (34) threaded periphery into the tapped hole in the fixed grip (29).

35. Rotate the detent plunger release knob (35) to the engagement position.

4T5. Description of the right- training handle assembly. The right training handle assembly operates the change of power mechanism by the movement of the revolving grip (3, Figure 4-44) and its interconnection with an appropriate mechanism in the eyepiece skeleton assembly (Figure 4-28). It is further interconnected by shifting wire tapes to the change of power mechanism in the skeleton head assembly (Figure 4-17) for changing from high-power to low-power magnification and vice versa. The right training handle assembly is similar to the left training handle assembly, and the variance of similar parts is described only briefly. Figure 4-44 shows the right training handle assembly. All bubble numbers in Sections 4T5, 4T6, 4T7 refer to Figure 4-44 unless otherwise specified.

a. Revolving grip. The revolving grip (3) is made of the same material and diameter as the left revolving grip (2, Figure 4-43), except that it is longer, and is provided with an undercut shoulder at the inner end. This shoulder has two graduated index lines, the upper has the letters H.P. engraved below it, while the lower has the letters L.P. engraved above it. These two graduated lines, when in coincidence with the stationary index lines on the fixed grip (2), visually indicate the power being used by the observer. A power indicating screw (27) is inserted in this undercut shoulder section to indicate low power when in coincidence with a similar power indicating screw (27) inserted in the fixed grip (2). When these screws are separated, the indication magnification is high power.

The counterbored section in the inner end is shallower in depth and receives the revolving grip inner collar (6), while the counterbored section in the outer end is the same as that which receives the revolving grip outer collar (4) and the revolving grip end cap (1).

b. Revolving grip outer collar. The revolving grip outer collar (4) is identical to the left revolving grip outer collar (3, Figure 4-43), and serves the same purpose and function in the outer end of the revolving grip (3). It is secured to the revolving grip shaft (9) with a lockscrew (13) in the same manner.

d. Revolving grip inner collar. The revolving grip inner collar (6) is made of the same material and has the same external diameter, reamed hole diameter, and counterbored section diameter and depth as the left revolving grip inner collar (5, Figure 4-43). It differs in length and has no tapped hole for an index ring actuating screw (22, Figure 4-43). The counterbored section receives the projecting shoulder and semicircular section of the fixed grip outer collar (5) and the segment stop (7) on the opposite side of the semicircular projecting section which is secured to the revolving grip shaft (9) with two lockscrews (24).

e. Fixed grip. The fixed grip (2) is almost identical to the left fixed grip (29, Figure 4-43) except that it has no tapped hole for the insertion of a detent plunger assembly. The undercut shoulder is provided with a graduated stationary index line and a power indicating screw (29) at assembly.

The counterbored section in the outer end carries the fixed grip outer collar (5) of a press

f. Fixed grip outer collar. The fixed grip outer collar (5) is similar to the left fixed grip outer collar (4, Figure 4-43) except for the length of the undercut shoulder. The semicircular section is provided with two segment stop adjusting screws (28) and two adjusting screw lockscrews (26) in the same manner. The adjusting screws (28) contact the segment stop (7) attached to the revolving grip shaft (9) for correcting insufficient or excessive travel of the segment stop (7) in relation to the high and low-power graduated index lines.

g. Revolving grip shaft. The revolving grip shaft (9) is identical to the left revolving grip shaft (30, Figure 4-43). This shaft is a sliding fit in the reamed hole in the revolving grip outer collar (4) and the inner collar (6). It has a tapped hole near its outer end to receive the threaded section of the lockscrew (13) which is inserted in a clearance hole in the revolving grip (3) and outer collar (4) for the manipulation of the shaft upon the rotation of the revolving grip (3).

The inner end of the shaft is a sliding fit in the reamed hole in the fixed grip outer collar (5), and extends the entire length of the fixed grip (2), a sliding fit into the large reamed hole in the handle hinge (15). The inner end of the shaft carries the large shoulder section of the outer bevel gear clutch shaft (8) of a press fit, and is secured with a taper pin (14).

h. Outer bevel gear clutch shaft. The outer bevel gear clutch shaft (8) is identical to the left outer bevel gear clutch shaft (8, Figure 4-43) serving the same purpose and function. It is secured to the revolving grip shaft (9) with a taper pin (14).

i. Segment stop. The segment stop (7) is similar to the left segment stop (7, Figure 4-43) and is approximately 114 degrees. It is secured to the revolving grip shaft (9) with two lockscrews (24). These lockscrews are inserted in countersunk clearance holes in the segment stop (7) and screwed into tapped holes in the revolving grip shaft (9) located in the counterbored section in the revolving grip inner collar (6). The segment stop is rotated with the revolving grip shaft (9) and contacts the adjusting screws (28) for high and low power,

j. Handle hinge. The handle hinge (15) is almost identical to the left handle hinge, (28, Figure 4-43) except that it is designed to be used by the opposite hand. The alignment support section carries the fixed grip (2) on its periphery and is secured with a lockscrew (13) in the same manner. However, the alignment support section of the handle hinge does not have the two countered sections for the main body stop (31) used in the left handle hinge (28, Figure 4-43) the square broached hole, and the opposite clearance hole.

k. Hinge bracket. The hinge bracket (29) is identical to the left hinge bracket (27, Figure 4-43) except that it is designed to be used by the opposite hand.

l. Pivot screws. The pivot screws (22) are identical to the left pivot screws (20, Figure

m. Outer bevel gear clutch collar. The outer bevel gear clutch collar (10) is identical to the left outer bevel gear clutch collar (9, Figure 4-43).

n. Inner and outer bevel gear clutches. The inner and outer, bevel gear clutches (16 and 17) are identical to the left inner and outer bevel gear clutches (14 and 15, Figure 4-43).

o. Handle detent plunger spring. The handle detent plunger spring (19) is identical to the left handle detent plunger spring (17, Figure 4-43), serving the same purpose and function in the handle hinge (15).

p. Handle detent plunger. The handle detent plunger (18) is identical to the left handle detent plunger (16, Figure 4-43), serving the same purpose and function in the handle hinge (15) and hinge bracket (29). It is secured with a retaining screw (20) in the same manner.

4T6. Disassembly of the right training handle. The right training handle is disassembled in the following manner:

1. Remove the lockscrew (13), unscrewing it from the revolving grip shaft (9), and carrying it out from the revolving grip (3) and outer collar (4) clearance holes.

2. Remove the revolving grip (3), sliding it off the revolving grip shaft (9), and carrying with it the revolving grip end cap (1), revolving grip outer collar (4), and the revolving grip inner collar (6).

3. Remove the two lockscrews (24) from the segment stop (7), unscrewing them from the tapped holes in the revolving grip shaft (9). Remove the segment stop (7).

4. Remove the lockscrew (13) from the fixed grip (2), unscrewing it from the tapped holes in the handle hinge alignment support section (15) and the fixed grip.

5. Remove the fixed grip (2) with the fixed grip outer collar (5), sliding it off the handle hinge alignment support section 15) and carrying it off the revolving grip shaft (9).

7. Swing the handle hinge to the extended position. Only in this position is there sufficient clearance for the removal of the outer bevel gear clutch (17) with the remaining assembly of the handle hinge (15) from the hinge bracket (29).

8. Remove the two pivot screws (22), unscrewing them from the tapped holes in the hinge section side walls of the hinge bracket (29). Remove the handle hinge assembly from the hinge bracket (29).

9. Remove the inner bevel gear clutch (16), sliding it out of the hinge bracket (29).

10. Remove the retaining screw (12), unscrewing it from the tapped hole in the outer bevel gear clutch shaft (8). Remove the outer bevel gear clutch (17) and the outer bevel gear clutch spring (11), sliding them off the square section of the outer bevel gear clutch shaft (8).

11. Rotate the revolving grip shaft (9) until the small end of the taper pin (25) is in line with the drift clearance hole in the handle hinge wall (15).

12. Place a drift punch of suitable size in the clearance hole.

13. Drive the taper pin (25) from the outer bevel gear clutch collar (10) and the outer bevel clutch shaft (8)

14. Remove the revolving grip shaft (9) and the assembled outer bevel gear clutch shaft (8) from the handle hinge (15).

15. Do not disassemble the outer bevel gear clutch shaft (8) from the revolving grip shaft (9). Leave them secured with a taper pin (14).

16. Remove the retaining screw (20), unscrewing it from its engagement in the keyway in the handle detent plunger (18) and the tapped hole in the hinge section rear side wall of the handle hinge (15).

17. Remove the handle detent plunger (18) and its spring (19) from the reamed hole in

18. The two segment stop adjusting screws (28) and the two lockscrews (26) are not altered during disassembly.

19. The power indicating screws (27) are not removed from the revolving and fixed grips (3 and 2).

4T7. Reassembly of the right training handle assembly. The right training handle assembly is reassembled in the following manner:

1. Lubricate lightly all rotating parts with. Lubriplate No. 110 as the reassembly procedure is followed.

2. Place the handle detent plunger spring (19) in the handle detent plunger (18).

3. Place the handle detent plunger (18) and its spring (19) in the reamed hole in the rear inner circumference of the handle hinge (15). Rotate the plunger until the keyway is located to the rear, and its detent point is lying in a horizontal plane so that the retaining screw (20) engages in the keyway.

4. Insert the retaining screw (20), screwing it into the tapped hole with its undercut shoulder engaging into the keyway in the handle detent plunger (18).

5. Place the assembled outer bevel gear clutch shaft (8) and the revolving grip shaft (9) in their respective reamed holes in the handle hinge (15).

6. Place the outer bevel gear clutch collar (10) on the outer bevel gear clutch shaft (8).

7. Align the taper pin holes in the outer bevel gear clutch shaft (8) and collar (10).

8. Insert and secure the taper pin (25), inserting it from the open hinge section side of the handle hinge (15).

9. Place the outer bevel gear clutch spring (11) on the outer bevel gear clutch shaft (8) and in the counterbored section in the outer bevel gear clutch collar (10).

10. Place the outer bevel gear clutch (17) on the square section of the outer bevel gear clutch shaft (8), with the reference marks in line.

12. Check the outer bevel gear clutch (17) for free spring movement.

13. Place the inner level gear clutch (16) in the reamed hole in the cored hinge section of the hinge bracket (29).

14. Holding the handle hinge assembly in the extended position, carry the outer bevel gear clutch (17) through the cored clearance section in the hinge bracket (29).

15. Check the reference marks of the inner bevel gear clutch (16) tooth with its mating reference mark in the outer bevel gear clutch (17). Engage the gear teeth of the inner and outer bevel gear clutches, carrying the hinge section of the handle hinge (15) over the hinge section side walls of the hinge bracket (29).

16. Apply downward pressure to the handle hinge (15) with the handle detent plunger (18) resting on the hinge section side wall periphery of the hinge bracket (29) compressing the handle detent plunger spring fully for the insertion of pivot screws (22).

17. Insert the two pivot screws (22) in opposite side walls of the handle hinge (15), check the reference marks, and screw them into the tapped holes in the high section side walls of the hinge bracket (29).

18. Secure both pivot screws (22) with the lockscrews (23). Insert these lockscrews in body clearance holes, and screw them into the tapped hole section in each hinge section side wall of the hinge bracket (29) from the inner side of the base. The lockscrews contact the pivot screw threaded sections.

19. Place the fixed grip (2) on the revolving grip shaft (9), sliding it on the alignment support section of the handle hinge (15).

20. Align the tapped lockscrew holes and insert the lockscrew (13). This lockscrew is screwed into the tapped hole in the fixed grip

21. Place the segment stop (7) on the revolving grip shaft (9). Secure it opposite the semicircular projecting section of the fixed grip outer collar (5) to the revolving grip shaft (9) with two lockscrews (24). These lockscrews are inserted in countersunk clearance holes in the segment stop (7) and screwed into tapped holes in the shaft.

22. Place the revolving grip (3) on the revolving grip shaft (9), carrying with it the outer and inner collars (4 and 6) and the end cap (1).

23. Insert the lockscrew (12), inserting it in the clearance holes in the revolving grip (3) and outer collar (4), and screwing it into the tapped hole in the revolving grip shaft (9).

24. Correct insufficient or excessive travel of the revolving grip power index lines by means of the two segment stop adjusting screws (28). The front adjusting screw corrects for low power, while the rear adjusting screw corrects for high power.

25. Make the correct adjustment of the low-power index line with the stationary index line on the fixed grip (2), by shifting to low power and then to high power. With an ear to the periscope, note the positive engagement click of the change of power mechanism in the skeleton head assembly. The adjustment should be made so that the adjusting screw has sufficient clearance to allow the revolving grip index line to come into coincidence with the stationary index line immediately after the change of power click is heard. This clearance should carry the segment stop (7) against the adjusting screw (28) after the positive engagement click has been heard. The high power adjustment is produced in, similar manner. Any adjustments necessary to the adjusting screws (28) for the low- and high-power index lines require the procedure outlined in Steps 2 and 3 for disassembly.

26. The change of power adjustment cannot be made until the shifting wire tapes are assembled to the skeleton head assembly (Figure 4-17) and attached to the shifting wire spindle

27. While making the change of power adjustment, it may be found that there is not a

In addition to these characteristics, another severe limitation is imposed on the submarine periscope: The ratio of the over-all length to the diameter of the tube must be large, from 40 to 100 to 1. And this must be accomplished without sacrificing field of view, magnifying power, brightness, or sharpness of image. The upper part of the periscope, in particular must be narrow and, in the case of the Type II periscope, this necessitates the addition of two one-power telescopes (five lenses). This undesirable addition of extra glass to the system is outweighed by the highly desirable reduction in diameter of the exposed part of the tube.

a. Telescope Systems. Inasmuch as the problem of the submarine periscope is solved by using two main telescopes with their axes coincident and their objective lenses facing each other, a brief consideration of simple telescopes is necessary.

1. Inverting telescope. A telescope is established when two lenses lying on the same axis are separated so that the back focal plane of the objective lens exactly coincides with the front focal plane of the eye lens. Thus, an object at infinity, or at a distance several hundred times the focal length of the objective, is imaged in the back focal plane of that lens. This image serves as the object for the eye lens and, lying in the front focal plane of the eye lens, is imaged at infinity. Thus, the telescope forms at infinity an image of some object which is also at infinity, and it might seem

2. Galilean telescope. The condition described in paragraph 1 is true if both lenses are positive, or converging, lenses. However, if one of the lenses is negative, the magnification still occurs according to the ratio of the two focal lengths, but the image is not inverted. Such an instrument is known as a Galilean telescope. One of these Galilean telescopes is used in the Type II periscope with its negative (shorter focal length) lens facing the incident light to produce the low-power magnification. In high power, the two lenses of the Galilean telescope are swung out of the field. See Section 4U8-c, paragraph 17, for the method of tracing rays through a reversed Galilean telescope (Figures 4-48 and 4-49).

4U2. Magnifying power. a. General. The magnifying power of any optical instrument is defined as the ratio of the size of the image seen through the instrument divided by the size of the image seen by the unaided eye. Thus, a magnifying power of unity, which the layman would term no magnification, means that the ratio equals one. The human brain, however, plays tricks on an observer, and when the eye views an image through a restricted aperture such as an eyepiece, if the magnifying power is just equal to one, the image seen through the instrument seems smaller than the image seen by the eye alone, although both images are identical in size. However, it has been determined that a

b. Simple telescope. In the case of simple telescopes, which make up the periscope, there are three other ways to define the magnifying power. One of these methods is given here and the other two are found in Section 4U9-c-5. M.P. = f₁/f₂, where f₁ means the focal length of the first lens the light rays pass through, and f₂ means the focal length of the second lens the light rays pass through. Of course, in a simple telescope there are only two lenses, the objective and the eye lens. The formula applies both to Galilean and inverting telescopes.

c. Periscope. The magnifying power of a periscope is simply the combined product of the powers of all of the component telescopes of the system, remembering that the power of any reversed telescope (that is, one with its short focal length lens toward the incident light) is the reciprocal of its normal power. It should be noted that each of the main telescopes in the Type II periscope employs an eyepiece system consisting of an eye lens and a collective lens, and in this case the power of the telescope must be determined by using the equivalent focal length of the eyepiece system and the focal length of the objective lens. See Section 4U9-b, for the method of determining equivalent focal length of a system comprising two lens. In the Type II periscope, the light rays emerge from the head prism to meet the following telescopic systems in turn.

A periscope in which it is possible to change from one power to another is called a bifocal or bipower instrument.

4U3. Field of view. The true field of view of any instrument is the angle between the extreme

True field of view =
Apparent field of view/ Magnifying power

a. High power. With the periscope in high power, the true field of view equals 48 degrees/6 equals 8 degrees, or 4 degrees on either side of the centerline of sight. The centerline of sight may be elevated or depressed as noted in Section 4U6, and shown in Figure 4-45.

b. Low power. With the periscope in low power, the true field of view equals 48 degrees/1.5 equals 32 degrees, or 16 degrees on either side of the centerline of sight. See Figure 4-45 Section 4U6. These figures, of course, do not include the full 360 degrees through which the periscope can be trained or the 74.5 degrees from full elevation to full depression of the altiscope prism.

c. Narrow 1.414 outer taper section. The extreme narrowness of the tube sections (second to ninth inclusive) is the most significant feature of the Type II periscope. The small outer diameter (1.414 inches of the outer taper section, Figure 4-15), which enhances the safety of the ship by lowering its visibility, is achieved, without reducing the true field of view, by the addition of two one-power auxiliary telescopes in the reduced tube sections of the periscope. The five lenses thus included bend the rays toward the optical axis and away from the tube walls. The addition of five extra lenses is undesirable because of the loss of light and the deterioration of image quality. However, these considerations are greatly outweighed by the decreased wake produced by the small diameter at the waterline.

4U4. Image brightness. The brightness of the image seen in the eyepiece of any instrument depends upon three things: a) the brightness of the object, b) the transmission efficiency of the instrument, and c) the relative size of exit-pupil-of-the-instrument to entrance-pupil-of-observer's eye. Since we can seldom control

a. Absorption-reflection losses. The amount of light that is absorbed in passing through an optical element depends upon the type of glass, and may vary from 0.06 of 1 percent to 0.10 of 1 percent. For our purposes, we assume that for each millimeter of glass path (measured along the axis of the periscope) 0.1 percent of the incident light is absorbed. Thus, in the Type II periscope at low power (total glass path = 268 mm) approximately 26.8 percent of the incident light is absorbed. At high power, the glass path is less because the Galilean system is out of the field (glass path = 258 mm) and the absorption is only about 25.8 percent. In applying this absorption loss to the reflection loss in order to determine the total loss, it is considerably simpler to employ the transmission which results from the absorption loss. Thus, at low power, the transmission effected is (100 percent - 26.8 percent =) 73.2 percent, and at high power, 74.2 percent.

The amount of light that is lost because of reflection depends upon the difference in index of refraction of the two optical media which are bounded by the surface causing the reflection loss. In most optical systems we find three types of boundaries, namely, a) air-to-crown glass, b) air-to-flint glass, and c) silvered glass surfaces. For our rough calculations of the theoretical values of reflection loss, we assume that in any periscope the loss of light at any silvered-glass surface is about 6 percent; and, on the basis of the Fresnel theory (see any optics textbook), we assume that for normal incidence at an air-crown glass surface, the loss is about 4.1 percent, and at an air-flint glass surface, about 5.6 percent. Again, in applying the above figures to those resulting from absorption in the glass, we employ the transmission (100 percent minus reflection loss in percent).

Since the Type II periscope has (low power) 20 air-crown surfaces, 16 air-flint surfaces, and 2 silvered glass surfaces, the transmission that would result if the only losses were due to reflection at the various surfaces is found as follows:

Transmission =
(1,000 - 0.041)²⁰ X
(1.000 - 0.056)¹⁶ X
(0.94)²

The following table is a comparison of theoretically and actually measured values for glass that has not been coated, and also the values for glass that has been coated with a magnesium fluoride evaporated film. This film is only a few millionths-inch in thickness on each glass surface, hence, does not appreciably affect the refraction of the light rays.

b. Effect of papillary size. The amount of light that can enter an optical instrument depends upon the area of the entrance pupil which is proportional to the square of the diameter of the pupil. Neglecting the losses in the system, caused by reflection and absorption, the amount of light that can leave the instrument is proportional to the square of the diameter of the exit pupil.

It is apparent that four times as much light can pass through an exit pupil 6 mm in diameter as through an exit pupil 3 mm in diameter. However, the brightness of the image depends also upon the area of the entrance pupil of the observer's eye.

The smaller of these two factors is the limiting factor. If the exit pupil of the instrument is

The pupil of the human eye varies in diameter with the brightness of the light entering the eye, carrying from about 2 mm, in bright light to about 8 mm in dim light. When the pupil is fully open, the spherical and chromatic aberrations inherent in the eye's optical system cause a falling off in image sharpness. When the pupil is fully stopped down, the reduced resolving power of the ocular systems causes a blurring of the image. Consequently, the ideal diameter of the eye pupil is somewhere between these two extremes. Actual measurements have shown that it is about 4 to 5 mm. It should be noted that the exit pupil of the Type II periscope is just 4 mm in both high and low powers.

c. Relation between central and oblique brightness. Upon entering the periscope, light rays from object points on or near the optical axis travel in lines approximately parallel to the axis in passing from one component telescope to the next. Of course, inside each component telescope these rays converge toward and then diverge from their respective image points. On the other hand, light rays from object points lying near the edges of the field upon entering the periscope, travel between telescopes in cylindrical bundles that are not parallel to the optical axis. And inside each component telescope these oblique bundles converge toward and then diverge from their respective image points. Since the objective lens of the lower main telescope is well removed from the objective lens of the upper main telescope, it is apparent that a sizable departure of any bundle from parallelism to the axis causes all or part of that bundle to strike the tube walls and be absorbed. Thus, the brightness of the image at the margins of the eyepiece field is always less than the brightness of the image at the center of the field. However, the human eye is not too critical in this matter and if the marginal or oblique brightness is at least half the central brightness, it is accepted by the observer as uniform brightness.

4U5. Orientation of image. Since the two reflecting prisms (head and eyepiece) are arranged

4U6. Head prism. The letters HA which are included in the design designation of the Type II periscope, indicate that the head prism may be elevated to a high angle, and the periscope is so designed that the head prism is able to move the line of sight through a total angle of 84.5 degrees, that is, from -10 degrees (below horizontal) to +74.5 degrees (above horizontal). The limits of the field of view in both powers are shown in the following table, and Figure 4-45 shows the low-power and high-power fields of view at maximum elevation of the prism tilt.

4U7. Target ranging devices. a. Telemeter. Each large division of a telemeter lens corresponds to an angle of 1 degree at high power, and 4 degrees at low power. Each subdivision corresponds to an angle of 15 minutes at high power, and 1 degree at low power.

If the angle subtended by the extremities of a target at the observer (angular size) and the linear size of the target are known, the range can be computed.

Since the telemeter is calibrated in degrees of true field, it provides a means of measuring the angular size of a target. The space between successive degree calibrations for high or low power on the telemeter is equal to f X tan 1 deg, where f is the focal length of the lens or optical system forming the image on the telemeter in

high or low power as the case may be. Thus the telemeter can be used as a rangefinder. The waterline masthead height is independent of the bearing of the target. Since this height is known, it is used in finding the range; the length of the ship is used because its angular size varies with the ship's bearing. If the range is determined according to masthead height, a range determination based on the ship's length is different unless the course of the target is perpendicular to the line of sight of the observer. If two range determinations, one based on height and one on length, are made, the ratio of the two is a measure of the course angle. While a telemeter can be used to make range and course angle determinations as just described, it is not satisfactory for such determinations because of the great difficulty in taking a reading from the telemeter at both extremities of the target when the observer's ship is not stationary.

b. Stadimeter. The built-in split objective lens stadimeter overcomes the telemeter lens difficulty. The angular size of the target is measured by forming a double image so that

For convenience the light image is considered as being at the center of the field.

1. When the objective halves are in position so that the split objective lens functions as a single whole lens, the inter-objective pupil, that is, the cylindrical bundle parallel to the optical axis of the periscope, is converged in a point lying on the optical axis of the lower objective (and periscope) in the back focal plane of the lower objective lens.

2. When the halves are moved as shown in Figure 4-46, the optical axis of each half is displaced from the periscope axis an amount equal to the movement of each half. However, the axis of each half remains parallel to the periscope axis. The back focal plane of each half of the objective lens remains in the same plane as before splitting.

3. Each half now picks up a lesser part of the inter-objective pupil than it did before splitting. However, the part picked up by any half is focused to a point lying on the axis-of that half and in the back focal plane.

4. Consequently each lens half forms an image, removed from the center of the field by a distance equal to the movement of that objective half.

5. If each half, on moving, causes the center of the image it forms to move a given amount,

6. If the split objective lens is normal, or unsplit, it forms a single image of the target. The halves, on being moved to different positions, each form a complete image of the target, displaced by an amount equal to the movement of that half from the original position of the target image formed by the unsplit objective lens.

7. Therefore, it is obvious that if a target image is split so that the waterline of one image coincides with the masthead d the other image, the sum of the movements of the objective halves in opposite directions is equal to the actual linear height of the target image, that is, waterline to masthead height.

8. If the linear height of the target is known and the equivalent focal length of the entire optical system forming the image is known, the angular height of the image can be computed. That is, for any particular movement of the objective halves necessary to Inform the waterline and masthead split-images there is a corresponding angular height of target.

9. By means of a cam and appropriate mechanism, the movement of the objective

10. Since the oblique pupils are smaller in cross section than the central pupil, as shown in Figure 4-46, beyond a certain angle the oblique pupils fall on only one of the objective halves. Consequently, when the lower objective lens is split, no double image appears for that portion of the field. This accounts for the fact that splitting does not occur all over the field.

The chief advantages of the stadimeter over the telemeter are: a) the separation of the two images, at any stage of the separation, is independent of any movement of the observer's ship; b) range angle and course angle are available directly from scale dials for quick reading without computation.

4U8. Optical maintenance. a. Arrangement of optical elements (Figure 4-47, page 54).

2. Placed in image plane of upper auxiliary telescope. It is placed in the first real image plane of the periscope, so that the graduations appear to vibrate in unison with the image and observation is easier.

3. It should be noted that the equivalent focal length of the Ramsden eyepiece system in the 1.414 periscope just equals the EFL of the upper eyepiece lens of the Type III periscope. This establishes the necessary data for machining the cam grooves that actuate the split objective lens, and permits the same stadimeter mechanism to be used in both designs.

4. The objective lens of the lower telescope is split so that the two halves may be shifted

5. A total reflection prism which has a curvature ground on entrance and exit faces may be called a dioptric prism (that is, a prism with refracting power), or a double-convex right-angle prism. It serves two functions, namely: it deviates the optical axis from a vertical to a horizontal direction, and it produces convergence in the ray bundles that are diverging from points in the image plane just ahead of the dioptric prism. In this latter function, it acts as a collective or field lens for the special eyepiece system of the Periscope-Ramsden eyepiece.

6. The fixed polaroid must be aligned with index marks on mount.

b. Ray tracing optical diagram (Figure 4-47, page 54).

1. Any ray passing through the optical center of a lens continues in the same direction, that is, there is no bending by the lens.

2. Any cylindrical bundle of rays entering a lens is converged to a point in the secondary focal plane of the lens, not necessarily on the optical axis.

3. Any cone-shaped bundle of rays diverging from a point in the primary focal plane of a lens is converged to a cylindrical bundle.

4. The image of any object-point is the intersection, after passing through the lens, of all the usable rays from the object point.

These rules apply particularly to positive, or converging, lenses. By substituting converging for diverging and vice versa in the above four rules, they apply specifically to negative, or diverging, lenses.

5. Thus, referring to the ray tracing as shown in Figure 4-47, page 54, and to the table in Section 4U2-b we trace various ray bundles through the Type II submarine periscope by noting their behavior when passing through the various optical elements of the instrument:

a) Since the object is at infinity (or practically so), all the rays from any one point of the object arrive at the head window in a cylindrical bundle.

b) Since the head window is planes-parallel, it does not affect the direction of the ray bundles or the parallelism of rays in any one bundle.

c) Since the head prism has plane faces (entrance, reflecting, and exit), it produces no convergence or divergence in the cylindrical bundles. The head prism, however, does deviate the line of sight so that it travels along the optical axis down the periscope tube.

d) Upper auxiliary telescope. If the Type II is in high power (see Section 4U8-c-17 for ray tracing in low power) the Galilean telescope is swung out of the field, and the cylindrical bundles next meet the eyepiece of the upper

e) If the periscope is in proper adjustment, the plane surface (containing the scale) of the telemeter lens also lies in this image plane. Thus, the target image is superimposed on the telemeter lens, and the rays continue on down the tube as though they originated at image points in the plane of the telemeter lens. By virtue of the fact that these ray bundles are diverging from the plane surface of the telemeter lens, that lens has practically no converging effect on the bundles. It does perform, however, a collective action by deviating the direction of each entire bundle. It produces zero deviation in the one bundle which meets the lens at the optical axis; but it produces its maximum deviation in those bundles which meet it farthest from the optical axis. In other words, because of its unique position in the system (that is, in an image plane), this telemeter lens acts like a thin prism but not like a lens.

f) The objective lens of the upper auxiliary telescope is placed just one focal length from the telemeter, so that the ray bundles diverging from that image plane are converged by it to form cylindrical bundles that travel down the tube until they meet the next lens.

g) Lower auxiliary telescope. The objective lens of the lower auxiliary telescope receives the cylindrical ray bundles and converges them to converging bundles with vertices in the back focal plane of the objective, where the rays, in each bundle cross this plane at a point, and then diverge toward:

h) The eyepiece (lens) of the lower auxiliary telescope, which converges the bundles into cylinders, since the rays had previously intersected in the front focal plane of the eyepiece. Thus, as we expect, the lower auxiliary telescope receives cylindrical bundles of rays and after inversion, sends them on down the tube as cylindrical bundles of rays toward the next optical element.

i) Upper main telescope. The cylindrical bundles of rays meet the eye lens of the eyepiece system of the upper main telescope and are converged toward image points in the back focal plane of this lens.

k) The objective lens of the upper main telescope receives the ray bundles which are diverging from the plane above (which is the front focal plane of the objective) and transforms them into cylindrical bundles that travel down the tube to the next telescopic system.

1) Lower main telescope. The objective lens of the lower main telescope (assume the two halves are not displaced but form a circle) receives cylindrical bundles and converges them to image points in its back focal plane, which is also the front focal plane of the lower eyepiece system, since this is a telescope.

m) The rays diverging from this image plane are converged by the dioptric prism, which acts as a collective lens for the lower eyepiece. Since the image plane is closer to the collective than its own focal plane, the dioptric prism is unable to converge the various bundles enough to form cylindrical bundles, hence the ray bundles enter the next lens:

n) The eye lens of the lower eyepiece system, with a slight divergence, and the converging power of the eye lens (that is, the reciprocal of its focal length) is just enough to transform the ray bundles to cylindrical bundles.

p) Rayfilters and polaroids. The rayfilters and polaroids are also plano-parallel and do not affect the vergence or the deviation of the ray bundles, so that the observer's eye finally receives cylindrical bundles if his eye is normal. If the observer's eye requires, for example, a -1.5 diopter setting of the eyepiece unit, he still receives a sharp image on his retina with his eye relaxed. Such a setting would merely indicate that the observer's eye at rest saw most clearly those objects (or images) which were 2/3 of a meter distant from his eye.

q) Galilean telescope. When the Type II periscope is in low power, the reversed Galilean telescope is included in the system, following the head prism and preceding the eyepiece of the upper auxiliary telescope. Since the Galilean system is a telescope, it transmits cylindrical bundles if it receives cylindrical bundles. The action of a reversed Galilean telescope, however, may not be obvious, and its formation of images is discussed in the following drawings:

6. The rays shown in Figure 4-48 are parallel to each other (although not parallel to the optical axis) because they are traveling to the right from a single point (Q) of the infinitely distant object. This object point is not shown in the drawing.

The dashed line M to the left of the negative lens represents the secondary focal plane of the above lens, and this, of course, is the plane in which are imaged all infinitely distant

If a positive lens, as shown in Figure 4-49, is placed to the right of the above negative lens so that its primary focal plane coincides with the image plane shown, all points in this plane are imaged by the positive lens at infinity. This, then, is a simple telescope; the secondary focal plane of the objective (negative lens in this case, since it is first to receive the rays) coincides with the primary focal plane of the eyepiece (positive lens, since this is the one nearest the observer's eye). Since one of the lenses is negative, the image is neither inverted nor reversed, and the system is called a Galilean telescope. The reader should refer to the four rules listed in Section 4U8-c and study their application in Figure 4-49.

The Type II, Type III, and Type IV periscopes have a divergent meniscus optical glass

d. Method of removing parallax caused -by gas pressure. If the image seen through an optical instrument is not sufficiently sharp, the system is said to be out of focus. If the image is sharply defined, however, it does not follow that the exact separation of the various lens elements has been established perfectly, for the human eye accommodates easily to slight divergence or convergence of the emerging ray bundles.

If the instrument carries one or more reticles, each reticle can be placed in its proper image plane with near perfection by means of parallax focusing. Since the Type II submarine periscope does have a reticle (the telemeter lens), the setting of this plane exactly in the first real image plane of the periscope is the problem we must solve with a great deal of care. When this is accomplished, the periscope is said to have no parallax.

Ordinarily this is a relatively simple matter inasmuch as parallax is detected readily by looking into the system and shifting the eye slightly (in a plane normal to the optical axis). If there is no parallax, there is no apparent movement of the image relative to the reticle, and if there is an apparent movement of image relative to the reticle, the image seems to shift

Achieving the condition of no parallax in the Type II periscope is complicated by the fact that the proper separation of the various lenses is established while the optical system is surrounded by air (at normal atmospheric pressure), while the periscope is to be used with the optical system surrounded by dry nitrogen at 7.5 psi above atmospheric pressure. Now the index of refraction of the various kinds of glass comprising the optical system of the Type II may be used to calculate the focal length (or refracting power) of each of the lenses, and if we assume that the index of refraction of air is 1.00000000 no appreciable error is obtained, even though the true index of refraction of air (at 15 degrees C, and at atmospheric pressure) equals 1.00027734.

Since we are concerned herewith the change that occurs when nitrogen is substituted for air and particularly when the nitrogen is under a greater pressure, we must compare the indices of refraction of the two media and determine what effect this difference in index has on the focal length of the various lenses in the system.

The index of refraction of the nitrogen (at 15 degrees C, and at 7.5 psi above atmospheric pressure) equals 1.00041968. By applying these two values in turn to the formulas for calculating focal length (when curvatures, index of the glass, and index of surrounding medium are known), it may be calculated that when a lens is surrounded

This means that if we remove parallax with the system in air and then gas the periscope to plus 7.5 psi, the telemeter reticle no longer lies in the image plane of the upper-auxiliary telescope eyepiece of the Type II periscope. It is necessary, in fact, before gassing the instrument, to increase the separation between the reticle and the preceding image-forming lens (in high power) by the factor 1.00038. This may be done by mechanical measurements; however, a much simpler method is to perform this shift optically, as follows:

If we use a target that is not infinity (when the system is in air) but is just 1,200 feet distant, the image formed by the upper-auxiliary telescope eyepiece falls slightly more than one focal length behind the eyepiece; in fact, it is just 1.00038 focal lengths behind the lens. Thus, when the parallax has been removed (high power) for a target 1,200 feet distant while the system is in air, after gassing the periscope, all targets that are infinitely distant are imaged exactly on the telemeter reticle; that is, there is no parallax.

After establishing the above separation between the telemeter reticle and the preceding image-forming lens, swing the Galilean system into the field (low power) and adjust the separation between the two lenses of the Galilean system so that a target which is only 35 feet distant is imaged in the plane of the telemeter reticle. Then after gassing the Type II periscope, there is no parallax (infinitely distant targets) either in high or in low power.

These targets may be outside targets that have been accurately ranged or, for shipboard use, they may be obtained optically by means of a distance collimator. See Section 4V8 regarding the Kollmorgen distance collimator.

The above adjustments compensate for the change in focal length of lenses preceding the telemeter. For lenses that follow the telemeter,

It is important to realize that the proper pressure for the nitrogen is from 5 to 7.5 psi. If the pressure is outside these limits there definitely is parallax in the instrument.

4U9. Glossary of optical terms.

a. Single thin lens.

1. Lens: any piece of glass or other optical medium that is bounded by two curved surfaces, usually spherical.

2. Center of curvature: the center of the sphere of which the lens face is a part.

3. Optical axis: the straight line connecting the centers of curvature of the two faces.

4. Thin lens: any lens thin enough that the primary and secondary nodal planes may be considered to coincide with each other and with the geometrical center plane of the lens itself.

5. Ray bundles:

a) Diverging bundles: all the rays that diverge from a single point of the object, or image, until they meet a refracting surface.

b) Cylindrical bundles: same as the diverging bundles except that the object point is so distant that only those rays that are parallel to each other succeed in getting into the lens.

c) Converging bundles: rays from a single object point that have been converged by a positive lens.

d) Paraxial bundles: ray bundles originating from an object point that lies on or near the optical axis of the lens.

e) Oblique bundles: originating from an object point that lies off the axis, near the margin of the field.

6. Object point: that point of the object which is under consideration.

7. Image point: that point of the image which corresponds to the object point under consideration; both object and image consist of an infinite number of points.

9. Image plane: plane normal to the axis, in which the image point lies.

10. Object distance: distance along the axis from an object plane to the center plane of a lens.

11. Image distance: distance along the axis from the image plane to the center plane of a lens.

12. Secondary focal plane: plane in which the image is formed when an object is at infinity.

13. Primary focal plane: plane in which to place the object in order to produce an image at infinity.

14. Focal point, primary or secondary: intersection of axis with appropriate focal plane.

15. Focal length: distance between focal plane and center plane of the lens.

16. Conjugate planes: planes so spaced that if the object lies in one, the image lies in the other. Thus, infinity and the secondary focal plane are conjugate. If the object distance is known in terms of the focal length of the lens instead of in inches or millimeters, it is quite easy to determine the image distance by the formula: if object distance equals n focal lengths, the image distance equals n/(n - 1) focal lengths.

17. Prism: any piece of glass or other optical medium that is bounded by two or more plane surfaces. Since ordinary prism faces have no curvature, the prism has no refracting power. The prisms does, however, produce a deviation in the incident bundles of rays.

18. Prism diopter: unit of deviation produced by a thin prism. If a prism produces a deviation of one centimeter in one meter of travel, it is said to have a power of one prism diopter, which is equivalent to an angle of ten mils.

19. Dioptric image: Newtonian terminology denoting any image formed by refraction, that is, by lenses.

21. Dioptric prism, or catoptric lens: derived name for the eyepiece prism of the 1.414 periscope, indicating that this collective prism both deviates the incident bundles and contributes convergence to these bundles.

22. Nodal planes: See Section 4U9, paragraph b-5.

b. System of two thin lenses. A system of two thin lenses may be considered as a single thin lens if the following three quantities are first calculated:

1. Equivalent focal length: the focal length of an imaginary thin lens that is equivalent to the focal length of the combination of lenses.

2. Back focal distance: distance from back surface of second lens to the secondary focal plane of the system.

3. Front focal distance: distance from front surface of the first lens to the primary focal plane of the system.

4. Method of calculating these three quantities:

Equivalent focal length =
(f₁ X f₂) / (f₁ + f₂ - S)

Back focal distance =
((f₁ X f₂) - (S X f₂)) / (f₁+f₂-S)

Front focal distance =
((f₁ X f₂) - (S X f₁)) / (f₁+f₂-S)

f₁ and f₂ denote the focal lengths of the first and second positive lenses respectively; and S is the separation between them in the same units of linear measure.

The rules for locating on a simple optical diagram, the positions of the front focal point (f), the back focal point (f'), and the primary and secondary nodal planes (N and N' respectively) of the above system are indicated below for the case where each quantity is positive. If any quantity works out toy be negative in value, take measurements in a direction opposite to that indicated.

FFD: measure from the surface of the first lens to the left a distance equal to the value found.

EFL: from the front focal point (F) measure to the right to locate the primary nodal plane (N), a distance equal to value found for EFL; from the back focal point (F') measure to the left to locate the secondary nodal plane (N'), a distance equal to the value found for EFL. Thus, the equivalent focal length of any combination of lenses is always the distance from either of the focal points of the system to the corresponding nodal plane.

5. Nodal planes: the primary nodal plane (N) of any system may be considered to be the plane of arrival for all rays entering the system, just as the secondary nodal plane (N') is said to be the plane of departure for all rays leaving the system. The object distance in the case of a system is always measured from the object plane to the N-plane; and the image distance is always measured from the N'-plane to the image plane.

6. Separation: when the distance between two lenses comprising a system has certain special values, optical systems with special characteristics are formed:

a) When the separation equals the sum of the two focal lengths, the system is a telescope.

b) When the two focal lengths are equal and the separation is equal to 2/3 of either, the system becomes a Ramsden eyepiece. This system is a form that has been modified to improve the color correction and is the type used in most telescopes and in main telescopic systems of periscopes.

c) When the focal length of the first lens equals three times the focal length of the second lens, and the separation equals 1/2 the sum of the two focal lengths, the system becomes known as a Huyghens eyepiece, which is commonly used in microscopes. The Huyghens ocular does not take a graduated reticle nearly so well as does the Ramsden eyepiece.

d) When the separation between the first and second lenses of any eyepiece equals the average of the two focal lengths, the system is well corrected for chromatic aberration.

1. Inverting telescope: two positive lenses separated by a distance equal to the sum of their focal lengths. See Section 4U2, for magnifying power.

2. Galilean telescope: one positive lens and one negative lens separated by a distance equal to the algebraic sum (arithmetic: difference) of their lengths.

3. Periscope: two main telescope systems aligned so that one optical axis coincides with the other, and placed with the two objective lenses facing each other. This arrangement satisfies the mechanical limitation of great length compared to the diameter of the instrument. It consists of a tube containing the above and provided at each end with reflecting surfaces, either mirrors or reflecting prisms, inclined at 45 degrees to the axis of the tube, so that an observer looking into one mirror (generally looking through an eyepiece) can see the objects reflected by the other mirror.

4. Magnifying power of any optical instrument:

Magnifying power =
(Size of image seen through instrument) /
(Size of image seen by eye alone)

5. Magnifying power of simple telescope: defined three ways:

a) MP =
(Focal length of objective) /
(Focal length of eyepiece*)

b) MP =
(Diameter of entrance pupil of instrument) /
(Diameter of exit pupil of instrument)

c) MP =
Apparent field of view (object nearest eye) /
True field of view (eyepiece nearest eye)

6. Magnifying a power of periscope: the power of a periscope is the product of the magnifying powers of all the component telescope systems. Reversed telescopes have powers that are reciprocal to their normal power; in other words, the Galilean telescope has a

* EFL if eyepiece is a system.

d. Ray tracing considerations. It may be helpful to consider the behavior of a bundle of light rays in traversing an optical instrument.

From each point of the object, light rays traveling in straight lines are radiating in all directions. We are interested, of course, only in those rays from a given object point that are directed in order to enter the first lens of our system. If this system is a telescope, the object is at infinity, or at a great distance, and the only bundles that enter the objective are those in which all rays are parallel to each other but not necessarily parallel to the optical axis of the instrument.

Upon passing through the objective of the telescope, these various cylindrical bundles are converged to their respective image points in the back focal plane of the objective lens; from these points they cross over and diverge toward the eyepiece lens. The eyepiece lens causes the diverging bundles to converge (since each bundle is coming from a single point in the front focal plane of the eyepiece) just enough to form cylindrical bundles again. Thus, the telescope forms at infinity an image of some object that is also at infinity, this image being magnified or minified in size and inverted or left erect, depending upon the type of telescope.

If we trace the rays through the Type II periscope, we need only consider that each telescope system in the periscope acts in the manner outlined above, receiving cylindrical bundles of rays from each object point (and there are an infinite number of such points in every object, however small the object) and emitting cylindrical bundles toward the next telescope in the system. It is suggested that the student form a periscope by suitably placing two auxiliary telescopes, for example, a 3x and a 5x. Keeping them coaxial, vary the distance between the two objective lenses to note the effect on image sharpness and on image brightness. The only limitation is found to be the brightness difference between the center and the margin of the field of view as seen through the eyepiece of the second telescope.

The design of the periscope is simplified by the fact that the human eye is not critical in the matter of comparing central and marginal brightness. In fact, if the margins are at least half as bright as the center of the field, the average observer agrees that the brightness is uniform throughout. If the marginal brightness is less than half the central brightness, the observer complains that the illumination is not uniform.