cury were perfectly elastic, a« 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
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