About the same time that Newton wrote, a Dutchman, named Huyghens, suggested that light comes from the sun in tiny waves, travelling across space much in the same way as ripples travel across a pond.  The only difficulty was to explain in what substance these waves could be travelling:  not through water, for we know that there is no water in space — nor through air, for the air stops at a comparatively short distance from our earth.  There must then be something filling all space between us and the sun, finer than either water or air.

And now I must ask you to use all you imagination, for I want you to picture to yourselves something quite as invisible as the Emperor’s new clothes in Andersen’s fairy-tale, only with this difference, that our invisible something is very active; and though we can neither see it nor touch it we know it by its effects.  You must imagine a fine substance filling all space between us and the sun and the starts.  A substance so very delicate and subtle, that not only is it invisible, but it can pass through solid bodies such as glass, ice, or even wood or brick walls.  This substance we call “ether.”  I cannot give you here the reasons why we must assume that it is throughout all space; you must take this on the word of such men as Sir John Herschel or Professor Clerk-Maxwell, until you can study the question for yourselves.

Now if you can imagine this ether filling every corner of space, so that it is everywhere and passes through everything, ask yourselves, what must happen when a great commotion is going on in one of the large bodies which float in it?  When the atoms of the gases round the sun are clashing violently together to make all its light and heat, do you not think they must shake this ether all around them?  And then, since the ether stretches on all sides from the sun to our earth and all other planets, must not this quivering travel to us, just as the quivering of the boards would from me to you?  Take a basin of water to represent the ether, and take a piece of potassium like that which we used in our last lecture, and hold it with a pair of nippers in the middle of the water.  You will see that as the potassium hisses and the flame burns round it, they will make waves which will travel all over the water to the edge of the basin,, and you can imagine how in the same way waves travel over the ether from the sun to us.

Straight away from the sun on all sides, never stopping, never resting, but chasing after each other with marvellous quickness, these tiny waves travel out into space by night and by day.  When our spot of the earth where England lies is turned away from them and they cannot touch us, then it is night for us, but directly England is turned so as to face the sun, then they strike on the land, and the water, and warm it; or upon our eyes, making the nerves quiver so that we see light.  Look up at the sun and picture to yourself that instead of one great blow from a fist causing you to see starts for a moment, millions of tiny blows from these sun-waves are striking every instant on you eye; then you will easily understand that his would cause you to see a constant blaze of light.