432 THE OUTGO OF ENERGY 



Myopia and hypermetropia will be further 

 considered under refraction in the eye. 



Chromatic Aberration. The velocity of the 

 homogeneous spectral rays composing white light 

 is believed to be the same in a vacuum and in 

 gases, but differs in transparent liquids and solids. 

 The front of the light wave strikes the refracting 

 surface obliquely. As the wave front enters the 

 medium, its speed lessens. Thus the part of the 

 front which enters first travels in the refracting 

 medium at a speed less than the remainder which 

 has not yet entered. The wave front is therefore 

 bent towards the retarded portion. The shorter 

 the wave length, i. e. the greater the wave num- 

 ber, the slower will the wave advance in the 

 refracting medium. Hence the wave front of the 

 violet ray moves more slowly in the medium 

 than the front of the red ray, and is therefore 

 bent more from its course. The reverse of this 

 process takes place when the wave emerges from 

 the refracting medium. The violet rays are there- 

 fore more refrangible than the red, and on enter- 

 ing a refracting medium pursue a different path. 

 Thus each spectral ray passing through a lens 

 has its own principal focus. In other words, the 

 images for the several spectral colors do not coin- 

 cide precisely. The order in which the refracted 

 spectral rays cross the principal axis is that of 



