364 



ON LIGHT. 



The light then which this wave conveys will be incident 

 perpendicularly on the surface E H, or in the direction of 

 the lines B F, c G, and these lines continued to K and L, 

 on the lower surface, would be the course of the rays B F, 

 c G, supposing them to undergo the ordinary refrac- 

 tion. Considering now the extraordinary; suppose the 

 portions E F, G H of the surface screened, and only the 

 portion F G of the wave allowed to enter. This on strik- 

 ing the surface, will excite at every point over its whole 

 extent a luminiferous vibration, which will be propagated 



\ 



Fig. 14. 



within the crystal in a spheroidal wave, having its shorter 

 .axis parallel to that of the crystal : and all these spheroids 

 being equal and similar, the plane which touches them all, 

 and which is, in effect, the extraordinarily refracted plane 

 wave within the crystal, will advance parallel to the sur- 

 face E H. Suppose it arrived at the other surface I M, 

 and let N o be the points of contact of that surface with 

 the spheroidal elementary waves whose centres are F and 

 G at that moment. Then will N o be that portion of the 



