THE REFRACTING TELESCOPE. 



177 



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 direction of their fronts, so that, if 



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 dis- 

 turbance. By locating the vibrating ball at random on the surface of 

 the mercury, it will also be seen that, unless the concave wave-fronts 

 are truly circular, they will not run to a single point of great agita- 

 tion, 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 no- 

 where 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 sur- 

 faces of revolution elliptical in section, called a lens. As already stated, 

 the light-waves are retarded during their passage through the body, and 



VOL. XXTIII. 12 



