1A1. Historical note. The periscope is the eye of the submarine. It was invented and developed solely for the purpose of providing a means to view the surface without fear of detection by surface craft. While it is primarily simple in principle, actually it is a complicated piece of apparatus. It is probable that all the navies of the world have similar instruments with only minor variations.

The earliest submarines were built without provision for periscopes and therefore, when submerged, were forced to grope their way blindly.

In 1854 Marie Davey, a Frenchman, designed a sight tube for a submarine. This tube contained two mirrors, one above the other, held at a 45 degree angle and facing in opposite directions. These, while providing some degree of sight to the submerged vessel, were faulty at best and, in 1872, prisms were substituted for mirrors.

Before the War Between the States, the submarine had not had a place among the ships of naval warfare. An American, Thomas H. Doughty, USN, was the inventor of the original periscope. Doughty's invention was not the result of study and research but rather the result of necessity. During the campaign of the Red River, while he was serving aboard the monitor Osage, Confederate cavalry, from the banks of the river, kept up a steady series of surprise attacks upon the Union vessels which had no way of seeing over the banks. This led Doughty to seek some new method of watching the shores. He took a piece of lead pipe, fitted it with mirrors at either end, and ran it up through the turret. This makeshift periscope provided sight for the crew of the Osage, enabled them to annihilate approaching Confederates, and practically freed her from further attack.

The earliest periscope, other than a collapsible one designed late in the nineteenth century by Simon Lake and known as an omniscope or skalomniscope, was a fixed tube. Soon, however, provision was made to allow the tube to be raised and turned by hand. This was fairly satisfactory

  when the boat was traveling at a low rate of speed but, with increased speed, the pressure was apt to bend the tube and throw the image out of line. Improved design resulted in a double tube, the outer to resist pressure and the inner to house the lens systems.

One of the biggest difficulties with the periscope in its infancy was that the rotation of the upper prism caused the image to be seen upside down. This has been corrected in the design of the instrument.

The Germans were responsible in large measure for the improvement of the modern periscope but, in spite of the advances made in the development of the instrument, the basic principle is still the same, the reflection of objects through mirrors or prisms arranged in a tube.

1A2. Periscope function. The essential function of a periscope is to give an officer conning a submarine a view of the surrounding horizon while his vessel remains submerged. To accomplish this, it is necessary that the periscope be long enough to extend beyond the surface, and that means be provided to deflect the horizontal rays of light first in a downward direction, and then horizontally to the eye of the observer. In addition, the part of the periscope which is to be above water must be as inconspicuous and streamlined as possible; for this reason the periscope is made in the form of a long narrow tube.

1A3. Periscope nomenclature. To insure a uniform method of designating periscopes on submarines, a standard system of nomenclature is used in all correspondence, specifications, and plans relating to such instruments.

The periscope nearest the bow is called No. 1 Periscope, regardless of whether it is of the altiscope type or whether it is installed in the conning tower. The next periscope aft of No. 1 Periscope is called No. 2 Periscope, and the next periscope aft of No. 2 is called No. 3 Periscope. The terms forward, middle, and after periscopes or 1st, 2nd, and 3rd periscopes are not used.


1A4. Useful definitions. The term periscope is used generally to designate all types of instruments. However, it is used specifically to designate instruments that are designed for horizontal view only.

The term altiscope is applied to a periscope from which the upper prism has been omitted and the view is directly upward toward the zenith.

The term altiperiscope is applied to instruments having the combined qualities of altiscopes and periscopes, sometimes called altiscope-periscopes and sometimes alti-azimuth instruments.

The terms unifocal and bifocal are used to refer to instruments of single and double power, respectively.

The term night periscope is used to designate a periscope having both high light transmission and an exit pupil of large diameter.

The term attack periscope is applied to a periscope with a minimum diameter of head at the sacrifice of light transmission and diameter of exit pupil.

The term metrescope is used to designate a periscope designed primarily for determining ranges of objects.

The term azimuth circle refers to the graduated circle used for taking bearings with the periscope.

The term stabilized azimuth device refers to a device in which a vertical wire in the field of the periscope is held gyroscopically in a fixed position in azimuth. The device is used in estimating the speed of an enemy ship.

1A5. Design designations of periscopes. Each separate or modified design of periscope is assigned a design designation, which is used in all correspondence relating to the periscope, in addition to the registry number of the periscope. The design designation is assigned by the Bureau of Ships and consists of the following parts in the order given:

1. A serial number for each design, assigned by the Bureau.

2. A letter indicating the manufacturer.

EKeuffel & Esser
BBausch & Lomb
SBarr & Stroud
ZNederlandsche Instrumentim Compagnie (Nedinsco)

3. A letter indicating the type of periscope. Letter Type.

ABifocal altiperiscope
HHigh-power altiperiscope
NNight or low visibility periscope

4. A number indicating the optical length of the instrument in feet to the nearest foot.

5. For a period, the letter T was added to indicate that the optics of the instrument had been treated to increase light transmission and improve definition. Since all periscopes in service have been so treated and new periscopes are furnished treated, this letter is not being included in recent design designations.

6. If the outer diameter of the upper portion of the reduced head section is less than 2 inches, a number representing the outer diameter of the upper part of the reduced head section in inches is added, separated from the preceding character by a diagonal mark.

7. If the instrument is an altiperiscope designed to permit view at any angle from the zenith to a point below the horizon, the letters HA are added.

8. As an example the following is quoted:

91(serial number)
A(bifocal altiperiscope)
40(optical length in feet to nearest foot)
T(treated optics)
1.414(outside diameter of upper part of reduced head section in inches)
HA(high angle)

Combined, this design designation reads as follows:


1A6. Marking of periscopes. The registry number of the periscope is conspicuously cut, or impress stamped, on the eyepiece end of each


periscope. It is also stamped on detachable external fittings, such as the training handles.

An etched or engraved name plate of suitable corrosion-resistant material is secured by screws to the eyepiece box of each periscope, and contains the following data:

DESIGN __________________
REGISTRY NO. ____________
MFG. _________ by
Manufacturer's Name
Manufacturer's Address

The inspector's stamp appears on the name plate.

1A7. Principles of modern periscopes. Everyone has looked through the wrong end of a telescope, that is, an inverted telescope, and viewed a normal scene much reduced in apparent size. This apparent reduction takes place because the inverted telescope takes a wide angle of vision and reduces it into a narrower one in the eyepiece. This principle is employed in periscopes. Essentially, a periscope consists of a vertical tube with a head prism inclined to the horizon at an angle of 45 degrees, a reducing telescope, and, at the bottom of the tube, an enlarging telescope and a lower prism facing the head prism and parallel to and below it. The objectives of the two telescopes face each other.

Suppose that a periscope is to be constructed with a field of 40 degrees. If, at the upper end of the tube, a telescope is installed with a reduction of 20x, or 1/20, the field angle is narrowed by lenses to 2 degrees. This field angle passes through a 5-inch tube for a distance of 12 feet. Now, if at the lower end a magnifying telescope of 20x is installed, the lenses of this telescope take the field angle of 2 degrees and expand it to 40 degrees.

If astronomical telescopes are used, the upper telescope inverts the image and the lower telescope reinverts it, so that the image appears erect to the observer. The distance between the objectives, about 12 feet, plus the lengths of the

  two telescope systems enable the periscope to attain sufficient length, for example, 27, 30, 34, or 40 feet.

If the periscope is to magnify the image, it is necessary either to decrease the reduction of the image by the upper telescope or to increase the magnification of the lower telescope. For example, if a magnification of 2x is desired, the upper telescope may be so changed that the field angle is reduced to only 1/10 of the original field angle, while the lower telescope remains unchanged; the magnification would then be 1/10 X 20, or 2x. Or the upper telescope may remain unchanged at 1/20 and the magnification of the lower may be increased to 40x: Then the final magnification is 1/20 X 40, or 2x, as before. However, the latter plan has the disadvantage of reducing the illumination. Since the size of the exit pupil is equal to the diameter of the objective divided by the magnification, the exit pupil is reduced if the magnification is increased.

1A8. Limits of periscope design. It is seen from the preceding section that there are definite limits in periscope design. The vital factors, as in a telescope, are: 1) length of tube, 2) diameter, 3) illumination, 4)magnification, and 5) size of field. If a periscope favoring any one of these factors is to be produced, such favoring can be only at the expense of the other factors; hence, the final design generally is a compromise.

1A9. Examples of periscope design. The following requirements are for periscopes which have been used in submarines: field, at least 40 degrees to 45 degrees; magnification, between 1.2x and 1.5x; exit pupil, at least 5 millimeters in diameter; length, not specified; external diameter, 5 inches; thickness of walls, about 1/4 inch. Let us find possible periscope lengths under these conditions for the two magnifications given, 1.2x and 1.5x. The inside diameter of the tube is 5 inches minus 1/2 inch, or 4 1/2 inches. The lens, lens-holding ring, supporting tube, and so forth take up another 1/2 inch of diameter, leaving about 4 inches free for the objective.

4 inches = 101.6 mm, which is close to 100 mm

In order to obtain an exit pupil of 5 millimeters, the magnification of the telescope must be:

Diameter of objective / Diameter of exit pupil =
100 / 5 = 20x


Figure 1-1. Section through submarine with periscope elevated.
Figure 1-1. Section through submarine with periscope elevated.

If the magnification of the final periscope is to be 1.2x, the reduction of the upper telescope must be:

20 / 1.2 = 16.67, or 16.67x

Since the field must be 40 degrees / 16.67, or 2.4 degrees = 2 degrees 24', this limits the length between the objectives of the two telescopes, since the entire beam of light must fall on the lower objective.

From Figure 1-3, it can be seen that the permissible length equals 2 / tan θ, where 2 is half the

  diameter of the lower objective lens in inches and θ is half the angle of beam. θ equals 2 degrees 24' / 2, or 1 degrees 12'.

log 2 = 10.30103 - 10

log tan 1 degree 12' =
(8.32112 / 1.97991) - 10

antilog 1.97991 = 95.58 inches =
7 feet 11 1/2 inches

The upper and lower telescope systems enter into the total length, and if it were possible to increase the focal length of their objective lenses

Figure 1-2. Detail of encircled section in Figure 1-1.
Figure 1-2. Detail of encircled section in Figure 1-1.

indefinitely, the periscope could be lengthened. Increasing this is limited, however, by the same considerations of diameter and cannot exceed the same length; that is, about 7 feet 11 1/2 inches for each telescope system. Hence, the total possible length is roughly 3 times 7 feet 11 1/2 inches, or about 23 feet 10 1/2 inches. Since this length is greater than is required, the diameter of the periscope may be reduced, the magnification increased, or the size of the exit pupil increased without sacrifice.

If the magnification is to be 1.5x, the reduction of the upper telescope must be:

20 / 1.5 = 13 1/3x

For a field of 40 degrees, the angle of beam is:

40 / 13 1/3 = 30 degrees

The inter-objective distance is:

log 2 = 10.30103 - 10

log tan 1 degrees 30' =
(8.41807 / 1.88296) - 10

antilog 1.88296 = 76.37 inches = 6 feet 4.4 inches

  The total length possible is 3 times 6 feet 4.4 inches, or 19 feet 1.2 inches.

To increase the length of tube beyond these limits, more telescopes may be placed in the tube. If astronomical telescopes are used, two more must be employed to keep the image erect, making a total of four telescope systems. One Galilean telescope could be used. The objection to adding more telescopes lies in the fact that each lens through which the beam must pass absorbs light, and if more are added, the illumination is seriously reduced.

Figure 1-4 shows a periscope designed as a straight instrument, and Figure 1-5 shows it with prisms introduced. The prisms may be placed at any point where the angle of the rays does not exceed the critical angle which results in total reflection. In this particular case, the prisms are placed at the focal planes. Both periscopes produce an erect image, since the two astronomical telescopes and the two prisms counteract each other in inverting the object. Prisms should not be placed exactly in a focal plane. Doing so is faulty design, since any minute imperfections

Figure 1-3. Example of periscope design.
Figure 1-3. Example of periscope design.
Figure 1-3. Example of periscope design.
Figure 1-4. Example of periscope design.
Figure 1-3. Example of periscope design.
Figure 1-5. Example of periscope design.

that may be present in or on the reflecting surface are reproduced as part of the final image, whereas a lens or glass plate which is not in a focal plane, or near one, may be dirty without affecting the resulting image. Periscope specifications often state that no lens or glass plate should be in or near a focal plane except the crosswire reticle, which must of necessity be placed in a focal plane.

Since the backs of the prisms, which are the reflecting surfaces, are silvered, the critical angle for reflection is raised to more than 20 degrees; thus the two eyepieces may be placed between the prisms and the objectives. Both forms of construction are used in various periscopes. However, the best position for a prism is at a point at which the rays are approximately parallel; in erecting telescopes, this point lies between the two erecting lenses.

The chief function of a telescope system in a periscope is to take an object appearing from the point of vision under narrow angular view, and produce it to the eye at a wide angle. The ratio of these two angles is the magnification of the telescope.

1A10. Altiscopes. The only difference between a periscope and an altiscope is that in an altiscope the upper prism is omitted and the view is directly upward toward the zenith. The field of an altiscope is 100 degrees. To obtain this field, some sacrifice must be made in other characteristics. The magnification is necessarily less than unity.

The only type of periscope used in the Navy today which permits observation of the zenith

  is the Type II design (Design Designations 89KA40T/1.414HA, 91KA40T/1.414HA, and 92KA40T/1.4HA built by the Kollmorgen Optical Corp., Brooklyn, N.Y., which is of the high-angle type. The prism has a maximum elevation of the line of sight above horizontal of 74.5 degrees. The entire sky is observed with the line of sight set respectively at 14 degrees, 44 degrees, and 74.5 degrees or full elevation, giving complete zenith at the edge of the field in low power. The periscope is rotated 360 degrees in each zone with a minimum of overlap between the zones.

1A11. Types of periscopes. Periscopes under Bureau of Ships Specifications R20 P5 of 15 June 1940, are of the following types:

1. Type I. Outer diameter of taper section, 1.414 inches. The line of sight can be moved through all angles between 10 degrees depression and 45 degrees elevation.

2. Type II. Outer diameter of taper section, 1.414 inches. The line of sight can be moved through all angles between 10 degrees depression and 74 degrees elevation.

3. Type III. Outer diameter of taper section, 1.99 inches. The line of sight can be moved through all angles between 10 degrees depression and 45 degrees elevation.

4. Type IV. Outer diameter of taper section, 3.750 inches. The line of sight can be moved through all angles between 10 degrees depression and 45 degrees elevation. The periscope is designed for night use with an installed antenna array and waveguide for the attachment of an electronic range device.

1B1. General description. a. The materials and workmanship of both mechanical and optical features of Navy periscopes are the best throughout. Particular attention is devoted to the accuracy, durability, ruggedness, especially as regards ability to withstand excessive vibration, and finish of the periscope and of each of its component parts. In deciding whether to reject flawed, improperly or inaccurately finished, or otherwise defective optical parts in which the flaws or defects are of such nature that they do not offer any possibility of more than very slightly reducing optical efficiency and durability of the   instrument, the state of advancement of the manufacturing of optical parts at the time the parts in question were manufactured is taken into consideration. However, the final decision always rests with the Navy Department.

b. Metals used in the construction of periscopes, except where otherwise specified, are brass, bronze, nickel-copper alloy, or corrosion-resisting steel. The balls of the hoisting yoke are made of stainless or corrosion-resisting steel. Carbon steel may be used for ball-bearing races and balls, springs, and small parts which must be


hardened. Carbon steel is not used for parts exposed to salt water. Carbon steel parts external to the sealed portion of the periscope are cadmium plated. Aluminum or aluminum alloys are used only in parts where lightness is essential, provided such parts are within the sealed portion of the periscope, and specific approval has been given by the Bureau of Ships.

c. The highest standards of mechanical construction are required, especially with respect to the hermetical tightness of the instrument and the arrangements for rangefinding, changing the magnification, operating the altiscope attachment, and focusing. Sharp corners or points which might be sources of chips or metal shavings during assembly and adjustment, or from vibration of the periscope, are avoided.

d. The construction of the periscope is such that the optics and internal mechanism may be easily disassembled and correctly reassembled, and the hermetical tightness of the instrument may be maintained.

1B2. General requirements for periscopes. When delivered to the Government, periscopes are completely assembled, including all parts and fittings. By means of the tests described below and by such other tests as the Government representative may require or conduct during the manufacture and after completion of the periscope, it must be demonstrated that the periscope meets the provisions of the specifications set up for its manufacture. The following requirements apply to all types of periscopes:

a. Hermetical tightness. The complete optics of the periscope, except rayfilters, are contained in a hermetically sealed tubular casing. Only the first surface of the head window and the last surface of the eyepiece window used in the optical system, are external to the hermetically sealed easing. The external casing is, in so far as possible, capable of withstanding without leakage the shocks, vibrations, and bending to which the instrument is subjected in service.

b. Tests of castings. The external casing and all castings forming part of the hermetically sealed portion of the periscope are given an internal air pressure test. When practicable, each casting is subjected separately to an internal air pressure test after the completion of all machine

  work. A part that shows signs of porosity on this test is rejected, unless, after effective steps have been taken by brazing, peening, and tinning or other means to remedy permanently the porous condition, and after the defective part has passed a successful internal pressure test, the acceptance of such part is specifically authorized by the Navy Department.

c. Cracking of metal under stress. In the selection of the material and method of manufacture of the various parts of the external casing, due regard is given to the danger of the development of porosity as a result of minute cracks that may occur in the metal when it is subjected to the stresses and vibrations encountered in service.

d. Joints in the external casing. All joints, in the external casing for the passage of moving parts, such as the operating gear for the power shift, altiscope, and focusing mechanism, are located below the hoisting yoke. All joints in the external casing which must be broken for overhaul, cleaning, or renewal of the optics or internal mechanism of the periscope, or for drying out the periscope, are located below the hoisting yoke, except in the Type I and Type II periscopes where one such joint is permitted at the upper end of the taper section.

1. The joints between the main body tube and the eyepiece box casting and taper section, and the joint between the taper section and head section are in accordance with Bureau of Ships Plans Nos. 306508 and 318815. Special provision is made in the case of screwed joints or joints held by screws to insure that the joint is not loosened by continued vibration. Setscrews and tap bolts, with lock washers or other locks, are used as necessary for this purpose. In installing such setscrews or tap bolts, special care must be taken not to drill entirely through the wall of the external casing of the periscope.

2. If it is necessary to drill screw holes completely through the wall of the external casing, the screws used in such holes are fitted with the utmost accuracy and, when practicable, are tinned and sweated in place. The threads of such screws engage only in threads in the wall of the external casing. However, this construction is avoided if possible. No holes are drilled through the main body tube or taper section.


3. Permanent joints which are not broken for overhaul, cleaning, or renewal of the optics or internal mechanism of the periscope are screwed joints. Before setting up, the screw threads are coated with a mixture of litharge and glycerin. Screwed joints are designed to provide an external shoulder about 0.20 inch in width. Such a shoulder requires a true and smooth finish. Gaskets for permanent joints are usually of soft annealed copper 1/32 inch thick. At the joint between the lower end of the main body tube and the eyepiece box, there is a triangular annular ridge on the shoulder 1/64 inch in height and approximately 1/16 inch in width at the base. The angles, including the apex, of this ridge are filleted. There is a corresponding triangular annular groove in the other face of the joint. In addition to the threaded part of the overlap of the permanent screwed joint between the main body tube and the taper section of the external casing, there is an unthreaded overlapping part. The latter part is located farther from the external seam of the joint than the threaded part, and the exterior surface of the inner overlapping part and the interior surface of the outer overlapping part are finish machined or bored to give the closest and tightest practicable fit. When practicable, these surfaces are slightly conical. This part of the joint is tinned and sweated, or coated with litharge and glycerin.

4. Joints which must be broken for overhaul, cleaning, or renewal of the optics or internal mechanism of the periscope are either screwed joints provided with a shoulder that seats against a gasket, or are secured by flush, fillister head screws of a noncorrosive material. The width of the shoulder of such a joint is at least 3/16 inch. Rubber gaskets of suitable thickness and at least 3/16 inch in width are inserted in all such joints. A triangular annular ridge is provided on one face of each such joint, and a corresponding triangular annular groove is provided in the opposite face of the joint. The faces of each such joint have a smooth and true finish, and a ground or scraped fit is preferred. In Type I and Type II periscopes, an exception to the foregoing may be made for one such joint at the upper end of the

  taper section, in which the width of the shoulder and the gasket width may be less than 3/16 inch, and the faces of the joint may be normal to the axis instead of finished with triangular grooves. The use of any such joint is subject to the specific approval of the Bureau of Ships.

5. Cover plates and retaining rings of joints secured by screws are of such thickness and the screw spacing is sufficiently close to guard effectively against any possibility of lack of tightness of the joint caused by springing of the metal between securing screws. However, screwed cover plates and retaining rings are preferred to cover plates and retaining rings secured by screws, especially in the case of joints which must be broken for overhaul, cleaning, and removal of the optics and internal mechanism of the periscope.

6. In the case of each joint which must be broken for overhaul, cleaning, or renewal of the optics or internal mechanism of a periscope, provision as far as practicable is made to enable the joint to be broken without undue difficulty. To prevent seepage of water between the threads of screwed joints of this character, the hermetically tight part of the joint is, when practicable, external to the threaded part. Special provision is made to guard against freezing of the threads of a screwed joint, resulting from corrosion of the metal caused by the seepage of salt water between the threaded parts of the joint. To provide for the easy removal of screwed cover plates, a hexagonal base is provided when practicable. This base conforms to the size of a United States standard hexagonal nut.

7. Joints in the eyepiece box casting of a periscope for the passage of moving parts, such as the operating gear for the power shift, altiscope, or focusing mechanism, are made in the form of stuffing boxes. Only motion of revolution is transmitted through a joint in the external casing.

8. Packed joints in the external casing of a periscope are thoroughly worked in before making the internal 150-pound test that must be made after assembly of the instrument. No further adjustments of these stuffing boxes are made after the successful completion of this test.


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