176 THE POPULAR SCIENCE MONTHLY. 



by the vibration of the molecules, in somewhat the same way as the 

 jelly might be agitated : by vibrating the wooden ball in its center, 

 each molecule as it swings sends an impulse or vibration through the 

 ether, which, traveling with equal velocity in all directions, forms as a 

 whole an expanding spherical wave-front, in shape like a quickly 

 blown soap-bubble, having the vibrating molecule at its center. 



The molecules of a hot body are in a state of intense vibration, and, 

 each being suspended in the substance of the ether, originate in it a 

 steady succession of these spherical wave-fronts, which, by one of the 

 fundamental principles of wave-motion in an elastic medium, do not 

 interfere with each other in the least, but each set of waves goes 

 straight on, as if every one of the other sets were not in existence. 



When light passes through a transparent substance, such as glass 

 or water, it is i)ropagated, not by the vibration of the molecules of the 

 substance, but by the vibration of the ether in which the molecules 

 are as it were submerged. This is proved by the enormous velocity 

 with which the vibrations are propagated within the substance, which 

 is immeasurably greater than the elasticity of the substance can ac- 

 count for. There are also other phenomena which lead to the same 

 conclusion, but which it is not necessary to allude to here. 



It has been found by direct measurement that the velocity of the 

 light-waves is less through transparent bodies than through space. 

 For some reason, the ether acts as if it were heavier within the body 

 than outside of it, being apparently condensed by the presence of the 

 molecules ; and the velocity of the waves is lessened by their passage 

 between the molecules of the transparent body, so as to produce an 

 effect similar to that produced on the velocity of waves on the surface 

 of water by the nearness of the bottom, where their velocity dimin- 

 ishes rapidly as the water grows shallower. 



Upon this simple fact, that the light-waves progress with less 

 velocity through transparent solid bodies than through space or air, 

 depends the complete explanation of the telescope. 



But, before considering the effect of this retardation of the light- 

 waves by their passage through transparent bodies, it is well to get a 

 definite idea of a wave-motion by observing one that is visible to the 

 eye. This can be beautifully done by the elliptical tank of mercury 

 roughly shown in Diagram 2 — the velocity of waves on the surface of 

 mercury being slow enough to be easily followed by the eye. 



The rim of the dish is elliptical ; the little ball to originate the 

 waves is constrained to vibrate at one focus of the ellipse, and it will 

 be observed that each time the ball makes a vibration a circular wave- 

 front, convex toward the direction of its motion, spreads out on the sur- 

 face of the mercury from the ball as a center, until, meeting the ellipti- 

 cal wall of the dish, it is changed by reflection to a circular concave 

 wave-front, which converges to its center, where the agitation of the 

 surface is much greater than anywhere else ; and, indeed, if the mer- 



