Periscopes - The Eyes of the Submerged Submarine

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. It is also equipped with a built-in stadimeter for estimating the range and course angle of the target.

Although advances have been made in the development of the instrument, the basic principle is still the same today: the reflection of objects through mirrors or prisms arranged in a tube.

USS Bowfin, as did most other submarines of the period, carried two periscopes. The one nearest the bow is called No. 1 Periscope, and the one aft is called No. 2 Periscope. On Bowfin, the No. 1 scope is the night scope, which has a large head, giving it both high light transmission and a large diameter exit pupil. It is 36 feet long and has a net weight of 2,000 pounds. The No. 2 scope is the attack scope, so called because of the small diameter of the head so as to be less easily seen by the enemy, and to leave a smaller wake, or feather, in the water from the moving submarine. This is at the sacrifice of light transmission and the diameter of the exit pupil. It is 40 feet long and has a net weight of 2,000 pounds.

Periscopes used on American submarines during World War II had definite limits in design. The vital factors were:

1. length of tube

2. diameter

3. illumination

4. magnification

5. size of field

If a periscope favoring any one of these factors was produced, such favoring was only at the expense of the other factors; hence, the final design generally was a compromise.

Most information taken from Submarine Periscope Manual, June 1946.