When we were talking about submarines in an earlier chapter, I took for granted that every boy and girl knows what a periscope is. Then we went on to talk about the torpedo which the submarine used, and the explanation of the exploding torpedo led us to think about explosive mines and mine laying and sweeping, but I can imagine some boy or girl wishing to ask how the periscope works.

It goes without saying that an ordinary telescope is of no use to the man in the submarine so long as he keeps under the water. If he did have a long telescope reaching above the surface, he would only be able to see the sky. Why? One boy suggests that it is because you cannot see round a corner, and another says it is because light travels in straight lines.

Suppose you were standing at a corner of a building which formed the corner of a street. You could not see what was going on round the corner, for the light reflected from the people and objects round the corner travels only in straight lines, and therefore could not enter your eyes because of the obstructing wall. But perhaps this statement about Light travelling only in straight lines is a little mysterious to you. I think it will become quite simple if you once realise what Light is.

I think every boy and girl realises what sound is, or in any case have some idea about it. You know that when the dinner-gong is struck the whole gong shakes or vibrates, and that the quick to and fro motions of the particles of the gong set up waves in the surrounding air. You cannot see these air waves, but when they enter your ears they produce those sensations which we call hearing; you hear that the gong is vibrating.

Light is also waves but not air waves. If it were the air that carried those waves which we call Light, then no light could reach us from the far-distant sun, as the ocean of air does not reach to the sun; it reaches only a few hundred miles upwards. But there is a great ocean of nether which fills all space, and vibrating particles in the sun cause waves in this ocean of æther, and it is these nether waves which we call Light. You know also how we imitate the sun with lamps of different kinds, all capable of setting up waves in this ocean of nether in which we live.

Having once realised what Light is, it is easy to understand that the æther waves must travel out in straight lines. But one girl says that while she can understand that the sun and fires and lamps produce waves in the surrounding æther, she does not see how people and the things around us send out Light. I might answer her by asking if she has ever played with a ball, throwing it against a wall at some distance, and then tried to catch the ball as it came back from the wall. How does the wall manage to throw the ball to her? She says the wall did not really throw the ball, it merely came back off the wall. I might say that the objects around did not really produce æther waves, but merely sent back or reflected æther waves which fell upon them. If the girl were a little older I should prefer to use a much better picture of what really happens, but for the present this will help her to think of Light being reflected from different objects to our eyes.

If we go back to the building at the corner of the street it is not difficult to realise a means of seeing round the corner. One boy suggests that you have merely to put your head out a little to catch the æther waves coming from the different objects, but that would not be seeing round a corner. It would be looking direct at the objects. The man in the submarine cannot push his head above the water to catch the æther waves which are being reflected from ships on the surface. In making our experiment at the corner of the building we must remain round the corner. I think you can all guess what we must do to see what is going on round the corner.

We must take a mirror and place it at an angle so that the æther waves will strike the mirror and glance off at an angle and thus reach our eyes. It was a straight line from the objects to the mirror, and it was a straight line from the mirror to our eyes. With the aid of mirrors the man in the submarine may see what is going on at the surface of the water above him, but in his case it cannot be quite so simple an arrangement as the single mirror at the corner of a building. The single mirror is used by the motormen on cars and buses, to see the back platform of their vehicles, so that they may not start when any passenger is boarding or alighting.

The long tube of the periscope is to hold the mirror up above the water where the æther waves of Light from surrounding objects may reach it. The little mirror then reflects the nether waves down the long tube to the lower end which is within the submarine. The officer does not wish to lie on his back to look up the tube, so he has another mirror at the bottom of the tube, which reflects the waves to his eye while he is in an erect position.

Perhaps you have seen simple periscopes such as just described. These were often shown in the shop windows of opticians during the Great War. Of course these periscopes were not intended for use in submarines, but to enable the soldiers in the trenches to see over the parapet without exposing their heads in doing so. Although the principle of the periscope for the submarine is exactly the same, the construction is different. Instead of ordinary mirrors there are little prisms of glass which are arranged to act as mirrors, and glass lenses are used to focus the light after passing through the prisms. One boy asks if the little prism or mirror at the top of the tube is facing the bow of the submarine to give a view of what is in front of the boat. This would be a very awkward arrangement, as an enemy might approach from either side or from behind without being observed, and if the man in the submarine wished to see round about, he would require to turn the whole boat in the direction in which he wished to look. How then does he get over this difficulty?

I think every boy and girl could suggest a very simple way out of the difficulty, enabling the man in the submarine to see what is going on in any particular direction. If any boys or girls cannot guess how this is done, let them hold a small mirror in front of them, and keeping the mirror just where it is, but turning it round to face one side and then the other side, it will be evident what a wide view of the surroundings may be got. And so the man in the submarine must be able to turn the little mirror at the top of the tube, so that he may face the bow, either side, or the stern. In this way he can spy the whole horizon.

In the lower part of the illustration facing the title-page, you see the Commander of a submarine looking into a periscope, and in the upper part of the illustration is the view which the man sees. The enemy ship which he is watching is not really sinking, but the sea is rough, and the waves partly hide the ship from the eye of the submarine, which is just above water. Looking at the illustration, some boy asks: How can there be telegraph poles out at sea? But these are not telegraph poles. They are marks made within the instrument itself, and you will notice there are other lines, which look as though they were to measure the height of the steamer.

The marks are really to enable the officer to measure how far the enemy ship is from him.

It will be of interest to make a simple experiment to help you to realise how difficult it would be to judge the distance of a ship when looking through a periscope. Take two common pins, and make one stick up in the centre of a pincushion, or on the top of a table, if you like. Place this pin just where you can reach it at arm's-length. Hold the second pin in your hand and bring the head of this pin right down on to the head of the other pin. You have no difficulty in doing this, but close one eye and try to do the same thing. You will find it so difficult that I think I could safely offer you a prize if you could hit the mark in the first three trials. Of course you must not feel about with the second pin until you touch the fixed pin; you must shoot forward your arm each time straight to the mark. The man in the submarine can only see with one eye, and that is why he must have some other means of judging the distance.

Someone asks if a submarine only uses its periscope when the boat is below the water. That is the chief use of the periscope, but it is also very useful when the submarine is on the surface, and for the same reason as the look out on a large ship is high up on the mast. The reason ought to be apparent, and yet someone asks why. One boy explains that it is for the same reason that you would climb to a height in order to get a good view, but this explanation receives some criticism. At sea there are no hills or other similar objects to obstruct one's view; all seems perfectly level. Of course the waves might obstruct your view of near objects, but why climb high up on a mast? Because the earth is a great ball, and the higher you get up the farther you can see along its curved surface. And so the periscope is of use as a look out when the submarine is on the surface.

Some boy wishes he could see through the periscope of a submarine, but I fear he may not have his desire fulfilled unless he becomes a naval officer. However, there is an instrument called a camera obscura which is practically a large periscope. There are not many camera obscuras to be found now. Of several that I knew in Scotland when I was your age, I can find only one now, and that is near to the Castle in Edinburgh. Upon the top of the hill is a high building with a look-out tower, and a room at the top is used as a camera obscura. Above the domed roof is a mirror and lens which reflect the image of the surrounding streets into the darkened room. This image falls on the white surface of a table in the centre of the darkened room, and there you see a cinematograph of all that is going on in the surrounding district.

On one occasion when I had taken some children to see the camera obscura, a lady expressed surprise that the picture was coloured. As it is merely the reflection from a mirror which one sees, the image is, of course, a coloured one. Most of you have looked through an ordinary camera, and have seen the coloured picture on the ground-glass screen below the dark cloth covering. When looking at a camera obscura it is just as though you were inside a huge camera.

The principle of the periscope is just the same as the camera obscura, but instead of throwing the image on a table, the officer looks through an eye-piece into the periscope. The mirror of the camera obscura can only face in one direction at a time, but by turning the mirror round into other positions, one can get a panoramic view of the whole neighbourhood.

Who invented the periscope? Some writers say that it is of French origin, but the Americans claim that the first periscope ever used was made by an engineer in the United States Navy. During the American Civil War (1864) a low-lying naval boat, called a monitor, was keeping guard on a river. As the boat went to and fro on this river the sailors were being shot one by one by some cavalry soldiers of the Federates, who could pick off the sailors without themselves being seen. The sailors, being low down on the river, were at a decided disadvantage. It was then that one of the engineers, Thomas Doughty, made what we now call a periscope. He fixed a long iron tube, like a mast, from the engine-room so that the upper end was right above the deck. With the aid of a mirror at the top and another down in the engine-room, Doughty could see what was going on along the high banks of the river. As soon as he saw the cavalrymen approaching, he signalled the gunners, who, turning the guns in the right direction, soon beat off the enemy. It was a mystery to the enemy how this monitor all at once seemed to have acquired a special gift of sight, and they soon learnt to give the boat a wide berth, or, in other words, to keep out of her way.

The French are quite correct in claiming the finished periscope, with its reflecting prisms and lenses, as their invention, and it may be that they knew nothing of this primitive American periscope. Both inventions might be said to be descendants of that very old invention which we call a camera obscura. The principle of that instrument was discovered accidentally by an Italian philosopher, who happened to see, on the wall of a darkened room, an inverted image of the view he usually got from that window. There chanced to be a small hole in the shutter which closed the window, so that the room acted as a great camera. But one young amateur photographer asks how there happened to be a glass lens at this hole in the shutter. There was no lens. I have beside me some very beautiful photographs taken by a camera without a lens: merely a small, very regularly shaped hole in a dark box. The lens merely enables us to focus or gather together more light.

When that Italian philosopher, who lived nearly four hundred years ago, added a lens to the dark shutter of his room, and placed the shutter on the roof instead of in the wall, and used a movable mirror to reflect the image down into the darkened room, we might say that he had really invented the first periscope.

On the last occasion when I visited the camera obscura the owner of the instrument told me that some sailors from the Navy had said that in some submarines the periscopes showed an image on a table just as in the camera obscura, but I think the sailors must have been mistaken, as I cannot see how sufficient light could be brought down the small tube of the periscope. The principle is the same, but the officer uses the periscope as a telescope, and the image of the surrounding scene on the surface is reflected down the tube from the mirror at the top to the one at the bottom, and then through an eye-piece directly into the eye of the officer.

There has been invented a panoramic periscope into which the officer looks in the usual way, whereupon he sees, in a ring round the central image, a complete view of all around the submarine, but this instrument is a much more complicated affair than the ordinary periscope.