We say wonderful because with it an observer in the conning tower of a modern submarine, though it is all but submerged, can see everything that is taking place on the surface of the water around the whole horizon.
The earliest trials at making a periscope were by using a simple arrangement of mirrors of an “L” tube and each at an angle of 45 degrees. The best way to understand its construction is to make one for yourself.
How to Make a Simple Periscope.—You can get a lot of fun out of this home-made periscope, and with it you can out-sherlock Sherlock Holmes, the great detective who was invented by Conan Doyle, for you can see around corners, over fences, and even back of yourself just as though you had a movable third eye, and without so much as ever being seen yourself.
Make a tube of wood or cardboard 2 inches square and 12 inches long, as shown in [Fig. 51]. Fit two square pieces of looking-glass into the corners of the tube at 45 degrees—that is half way between the vertical and the horizontal—and your periscope is finished and ready for use.
FIG. 51. HOW TO MAKE A SIMPLE MIRROR PERISCOPE.
How the Periscope Works.—When, now, you hold the long tube of your periscope in a vertical position that is straight up and down, and the light from an object, a person, or a scene at which the upper horizontal tube, or objective, as it is called, is pointed, strikes the first mirror, it bends the rays of light at an angle of 90 degrees, when the light goes straight down the tube as shown by the arrows in [Fig. 51].
When it strikes the other and lower mirror, the rays of light are again turned out of their path at an angle of 90 degrees, when they are reflected out of the lower horizontal tube which forms the eye-piece.
If now you will place your eye to this end of the tube, you will be able to look all around and see what you shall see. See?