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THE CHEMICAL ACTION OF LIGHT 37
which appear to widen out from a centre, and as they extend become gradually less prominent, until they finally disappear. If several stones are thrown at the same time into the water, each of them forms its own system of waves. These intersect each other in the most intricate manner ; and, although an apparent confusion of rings takes place, it is wonderful that none of them disturbs the other, and that each circle widens out regularly from its own centre, where the stone fell into the water.
If a handful of sand, which contains many thousand grains, is thrown into water, and if the attention be directed to the undulations of a single grain, it will be remarked that this one, without being affected by the countless other waves, widens out in a regular circle.
These undulations are one of the most remarkable movements in nature, taking place not only in water, but in a modified form in the air, where they occasion the propagation of sound. Fig. 10.
The peculiar feature of the undulatory movement consists in the fluid appearing to advance without really doing so. If, sitting on the side of a sheet of water, we see an undulation approach, it appears exactly as if the particles of water were approaching us from the origin of the movement.
It is easy to prove that this is an error by throwing sawdust or a piece of wood into the water. It dances up and down upon the ripples without moving from the spot. Indeed, the undulation is itself only an up and down motion of the particles of the water, and this movement is communicated further and further to the neighbouring particles.
Exactly in the same manner light spreads in undulations from a luminous body through the ether of space in all