3^4 



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 j-ays b f, 

 c G, supposing them to undergo the oj'dinary 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 

 '"xtent a luminiferous vibration, which will be propagated 



N 



M 



Fie 



T4. 



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 eftect, the extraordinarily refracted //<z;/^ 

 wai'e ivithin 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 

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



