Page:Popular Science Monthly Volume 28.djvu/187

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cury were perfectly elastic, as is the ether, the agitation at the center of the completely circular concave wave-fronts would be as great as at the origin of the disturbance.

We also see, from this experiment, that circular wave-fronts travel in a direction at right angles to the direct ion of their fronts, so that, if

PSM V28 D187 Circular wave changes when bouncing off the wall of an elliptical bowl.jpg
Diagram 2.

from any cause a wave-front becomes circular and concave toward the direction in which it is moving, it will run to a perfect center or focus, and at that particular place create a comparatively great disturbance. By locating the vibrating ball at random on the surface of the mercury, it will also be seen that, unless the concave wave-fronts arc truly circular, they will not run to a single point of great agitation, but only a confusion of cross-waves will result.

The same phenomena of wave-motion made apparent to the eye on the surface of the mercury are also true of light-waves: if from any cause the wave-fronts become spherical, and at the same time concave, toward the direction in which they are moving, they will also run to a center, and cause intense agitation at that particular point, but nowhere else.

Diagram 3 represents the effect produced upon the light-waves diverging with uniform velocity and spherical fronts, from a vibrating molecule, by passing through a transparent body, whose faces are surfaces of revolution elliptical in section, called a lens. As already stated, the light-waves are retarded during their passage through the body, and