The Modern Lenticular[25] Periscope.—The toy periscope which we have just described, and which we hope you will make and use, does not show what is going on at any great distance, and while this will not interfere with your pleasure of using it, it mattered very greatly when it was used as an eye for a submarine craft.
To be able to see farther led to the idea of using a telescope in connection with the mirrors in the tube, and a periscope of this kind was next made and tried out, and, let it be said, the results obtained were a decided improvement as against those where mirrors alone were used.
How the Telescope Is Made.—You may or may not know it, but an ordinary telescope, or spy-glass, as it is called, is made up of four lenses, as shown in [Fig. 52].
The purpose of the large lens in the front end of the tube—or object glass, to call it by its right name—is to gather in the light of the object and form an image of it. The small lens in the back end of the tube is used to magnify the image formed by the object glass.
Now, when a telescope has only an object-glass and an eye-piece, the magnified image of the object looked at is always upside down; this is the kind of telescope that astronomers use in their star work and the fact that the image is reversed doesn’t really matter anyway because the man in the moon looks about as well when he is standing on his head as he does when he is right side up.
FIG. 52. HOW AN ORDINARY TERRESTRIAL TELESCOPE IS MADE AND WORKS.
But when you want to look at objects here on the earth’s surface you want to see them as they are and not upside down. To rectify the image, which means to make the eye see the object as it is, two more lenses are placed in the tube, and so four lenses are used in all.[26]
About the Reflecting Prisms.—The next big improvement in periscopes came when a total reflecting prism was used in the place of the mirror at each end of the tube.