One of the gravest faults the pilots of the first submarines had to contend with was that they were not able to see where their craft was going.
But like everything else that is needed for the welfare or warfare of the human race, inventors got busy and began to scheme and to experiment, with the big idea of making an instrument which would do for the submarine what the eye does for the brain, and that is to look around with.
How the Eye of the Submarine Got Its Name.—Now, the eye of the submarine is called a periscope and before we go any further let us find out how it came to get this peculiar name for then we shall know more of what we are talking about.
The early Greeks used two words that were very common to them, and you, too, will use them, before you get through with this chapter, just as easily as they did. One of these words is peri which means around, and the other is scopeō which means to look; so peri plus scopeō means to around look, or, as we barbarians say it, look around—which sounds better to us. And there you have the roots of the word periscope.
The First Submarine Eye, or Periscope.—As in the beginning of all things, good and evil, the first attempts to make an eye for the submarine, or a periscope, were very crude, and as usual they were nearly, if not quite worthless.
But the periscope was no exception to the first law of invention, and that is that each attempt, however much of a failure it may have been in itself, had the germ of a useful device in it; and from out of all these efforts finally came that wonderful optical instrument, the periscope.
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.
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?
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.
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.
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.
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.
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.
The reason it is necessary to have a fixed outside tube is because the force of the water, when the craft is submerged and is speeding along under power, presses against the tube so hard that if only the inside one were used it would bind; as it is, there is no pressure on the inside tube and it can therefore turn freely at all times.
All recently built submarines have two periscopes, one of which leads into the conning tower and the other one runs down into the navigating room. Hence if one or the other is put out of commission by shell-fire, or otherwise, the submarine can still see and find its way about.
Gauging the Distance of an Enemy Ship.—An instrument called a telemeter (pronounced te-lem´-e-ter) is attached to the periscope near the eye-piece, and the observer can by looking into it measure the distance away of an enemy ship. This is done from the size of the image it makes on the eye-piece.
Without this instrument the whole submarine, as large, as wonderful, and as costly as it is, would be of small value, for by it the captain is able to set his torpedo director very accurately and hence to aim the torpedo so that it will make a sure hit.
The Latest Type of Periscope.—As the periscope just described, and which is still in use on submarines, has a very limited field of vision at any given setting—that is to say, only about ⅙ th of the horizon can be seen when the instrument is pointed one way—and as a periscope which would show the whole horizon at the same time was badly needed, a British firm of opticians set out to invent one.
The hardest part of the task was not to get a complete view of the whole horizon at the same time but to prevent the rays of light which form the images from getting mixed up with one another, or interfering, and so producing a blurred and indistinct picture.
The new complete-view periscope differs from the older style only in having a circular lens and prism; these gather in the light, bend the rays and project them on down through the telescope lens until they reach the lower prism when all of the images are reflected into the eye-piece where the observer sees it as a circular picture.
This new improved form of periscope is of great value, for it gives a safety-first view on all sides of the submarine at the same time and the observer does not need to keep turning the eye-piece, and this is of great value when a submarine is being closed in on by two or more enemy ships.
It is a well-known fact that a large number of submarine accidents have been caused by the limited range of view offered by the old-style periscope, and in some cases the undersea craft has been rammed or sunk by gun-fire from an enemy ship which she did not see.
The new periscope makes it impossible for a destroyer to creep up on the submarine without being caught in the act. Nearly all of the undersea craft now being turned out by all of the warring nations are fitted with the new 360-degree[27] vision periscope.
The Limited Use of the Periscope.—Notwithstanding all these latest improvements in the periscope, its use is quite limited, for it can be used only when the submarine is running awash or partly submerged, and since the tube of the instrument is only 20 feet high the distance to which a ship can be seen is about five miles.
When running partly submerged the tube of the periscope sticks up and out of the water about 10 feet, when the distance range of vision is then cut down to about two miles, for the curvature of the earth’s surface meets the line of sight and everything that is at a greater distance than this from the submarine is below the horizon and hence invisible.
Should an enemy destroyer get within firing range of the submarine and the captain of the latter craft wants to watch it, only the hood of the periscope is poked up above the water; but of course the distance range is again cut down.
The captain of a submarine has to contend with all these adverse features of the periscope even on bright, clear days and when the sea is calm; on dark and foggy days, when a heavy sea is running, the periscope is next to useless, for the mist and spray gather on the objective lenses and this makes it next to impossible to see anything.
Worst of all are the waves which break over the periscope, and this prevents a ship from being seen even if it is only a little way off. It is in rough weather that a submarine takes the longest chances; but to put behind him any danger that may be lurking hard by, the captain prefers to run undersea and come to the surface only when he has to.
A scheme to clear the moisture from the objective lenses is a device called a sprayer. It is made and worked so that an observer at the periscope can spray the lenses with alcohol. As water has a very great liking—or affinity, as it is called—for alcohol, and as alcohol evaporates almost instantly it carries the particles of water off with it, and this helps to make the seeing better.
The New Enemy of the Submarine.—A new enemy of the submarine has recently made its appearance—an enemy that will make it use a periscope of a new order.
This latest submarine destroyer is the airplane; and as the captain cannot now see directly overhead except when his boat is running light or awash, and the pilot of an airplane can see the submarine when it is submerged to a very considerable depth, it is easy for him to follow the undersea craft until she comes to the surface and then drop a bomb on her.
A story of a running fight between an airplane and a submarine would have put it in Col. Roosevelt’s Ananias Club a few years ago, but to-day it has all come to pass, and it looks now as if a good way to break the backbone of Germany’s ruthless warfare on the sea is to destroy the U-boats with a fleet of airplanes.