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THE CHEMICAL ACTION OF LIGHT
Light Waves. — Two sciences join hands to accomplish the wonders of photography. One is Optics, a division of Physics, and the other Chemistry. These alone are inadequate to fulfil the requirements of photography. ^Esthetical claims have to be considered ; and thus photography unites in itself the provinces of natural science and of the fine arts which at first sight seem remote and incapable of union. We shall consider first the principles of optics — that is, light — as the force which occasions the chemical changes in photography. We shall see that its chemical operations have not only become the basis of our art, but that they have played, and still play, a much more important part in the development of our planet.
We are aware of the existence of sun, moon, and planets. We know their distance ; nay more, we know their elements, though we are separated from them by millions of miles.
We are indebted for all this knowledge to waves of light which come to us from these far-off bodies. In order that such waves ma}' be transmitted from one place to another, it is necessan to assume the existence of some intervening medium.
Such a medium, to fulfil the requirements of a transmitter of light waves, must fill all space, have no weight, and yet be capable of acting in all respects like a perfect elastic, incompressible fluid. To this medium we give the name of The Ether.
If we throw a stone into water, waves are produced — that is, circles or rings of hills and valleys are formed,
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