FRESNEL'S THEORY OF DOUBLE REFRACTION. 213 



section of the cone made by the surface of emergence. This 

 is represented in the annexed diagram, in which NO is the 

 incident ray, aOb the cone of refracted rays within the 

 crystal, and aa'Vb the emergent cylinder. 



The minuteness of this phenomenon, and the perfect ac- 

 curacy required in the incidence, rendered it much more dif- 

 ficult to observe than the former. A thin pencil of light, 

 proceeding from a distant lamp, was suffered to fall upon the 

 crystal, and the position of the latter was altered with extreme 

 slowness, so as to change the incidence very gradually. "When 

 the required position was attained, the two rays suddenly 

 spread out into a continuous circle, whose diameter was ap- 

 parently equal to their former interval. The same experiment 

 was repeated with the sun's light, and the emergent cylinder 

 received on a small screen of silver paper, at various distances ; 

 no sensible enlargement of the section was observable on in- 

 creasing the distance. The angle of this minute cone within 

 the crystal was found to agree, within very narrow limits, 

 with that deduced from theory, the observed angle being 

 1 50', and the theoretical angle 1 55'. 



The rays composing the internal cone are all polar- 

 ized in different planes ; and the law connecting these 

 planes is the same as in the case of external conical refrac- 

 tion. 



