460 REFRACTION THROUGH PRISMS. 



cidence, and the light passing now into a less refracting 

 medium, the angle of emergence must be greater than 

 g h e, and h k will be the direction of the ray when it 

 leaves the prism. It follows that if the prism has the 

 position represented in the figure, the ray of light is 

 refracted downwards ; in general, a ray of light passing 

 through a prism of higher refractive power than the 

 surrounding medium, is refracted from the edge arid 

 towards the base b c of the prism. An object viewed 

 through a prism appears, on the contrary, nearer to 

 the edge and further from the base, for the ray is 

 bent downwards, and reaching an eye at k has the 

 direction h k', the eye receives the impression of 

 luminous rays proceeding along / h k, that is, it sees 

 the light of the candle at I. 



The deviation of a ray of light which passes through 

 a prism is the greater, the greater the refracting angle 

 of a prism, and the greater the refractive power of the 

 substance of which the prism is made. Thus in fig. 261 

 the prisms A and B are supposed to be both of glass, 

 but the refracting angle of the former is greater than 

 that of the latter, hence the deviation produced by A is 

 greater than that by B; again, C is a prism of water, 

 having the same refracting angle as A, but the devia- 

 tion caused by C is less than that caused by A, glass 

 being more refractive than water. 



Prisms of liquid substances can of course be formed 

 only by enclosing the liquid substances between 

 inclined surfaces of glass ; if the glass plates employed 

 for these purposes have perfectly parallel faces, the 

 rays of light are unaffected by them and suffer solely 

 the deviation due to the liquid. 



Sunlight which passes through a window may be 



