In physics the terms total reflecting means simply that all of the light that strikes a surface is reflected again without loss. While a mirror will reflect only a part of the light that falls on it, a prism will reflect all of the light that enters it.
FIG. 53. THE REFLECTING PRISM.
A prism is a three-sided piece of glass, if you forget to count the ends, as shown at A in [Fig. 53]. When a ray of light enters, say, the vertical side of the prism it keeps on going until it strikes the 45-degree side of it; this side reflects and bends it and it passes out of the horizontal side, as shown at B.
FIG. 54. HOW A MODERN PERISCOPE IS MADE AND WORKS.
The Construction of the Periscope.—Knowing now how a telescope is made and what it does and also knowing what a prism is and how it acts on light, all you have to do to understand the construction of a submarine periscope is to take a good look at [Fig. 54].
You will see that a prism is fixed in the upper end of the tube and directly back of the object-glass; that another prism is fixed to the lower end of the tube and back of the eye-piece; and that the rectifying lenses of the telescope are set between these two prisms. This picture also shows the path of the light through it.
These lenses and prisms are mounted in a tube about 4 inches in diameter and 20 feet long. A horizontal revolving hood is secured to the upper end of the tube and the horizontal eye-piece is fastened to the lower end of it; a wheel is also fixed to the lower end of the tube, so that the observer can turn the periscope completely around and so scan the surface of the sea in any direction.
The periscope tube is placed in a slightly larger and very strong steel tube, which passes through the deck of the conning tower and into the latter, and it is made watertight by means of a stuffing-box.