494 THE HUMAN BODY. 



incidence to that of the angle of refraction is always the 

 same for the same two media with light of the same wave- 

 length. When the first medium is air the ratio of the sine 

 of the angle of refraction to that of the angle of incidence 

 is called the refractive index of the second medium. The 

 greater this refractive index the more is the refracted ray 

 deviated from its original course. Eays which fall perpen- 

 dicularly on the surface of separation of two media pass on 

 without refraction. 



The shorter the oscillation periods of light-rays the more 

 they are deviated by refraction. Hence mixed light when 



FIG. 128. Diagram illustrating the dispersion of mixed light by a prism. 



sent through a prism is spread out, and decomposed into 

 its simple constituents. For let a x (Fig. 128) be a ray of 

 mixed light composed of a set of short and a set of long 

 ethereal waves. When it falls on the surface A B of 

 the prism, that portion which enters will be refracted 

 towards the normal ED, but the short waves more than the 

 longer. Hence the former will take the direction xy, and 

 the latter the direction x z. On emerging from the prism 

 both rays will again be refracted, but now from the nor- 

 mals Fy and G z, since the light is passing from a more to a 

 less refracting medium. Again the ray xy, made up of shorter 

 waves, wi 1 ! be most deviated, as in the direction y v, and 



