194 FRESNEL. 



During the long discussions which took place among 

 physicists on the mathematical law according to which 

 double refraction is produced in Iceland spar, the exist- 

 ence of the second ray was generally considered as an 

 anomaly affecting half the incident light ; the other half, 

 it was said, obeyed the old law of refraction laid down by 

 Descartes : the carbonate of lime, in its crystallized state, 

 then, enjoys certain particular properties, but without 

 losing those which all ordinary transparent media pos- 

 sess. All this was exact in the instance of the Iceland 

 spar, and it seemed as if it might without hazard be 

 asserted generally ; but in fact those who maintained 

 this deceived themselves. There are crystals in which 

 the principle of ordinary refraction is not verified ; and 

 in which the two rays into which the incident light divides 

 itself both undergo anomalous refractions, where the law 

 of Descartes does not indicate the course of either ray. 



When Fresnel for the first time published this unex- 

 pected result, he had as yet verified it only by the aid of 

 an indirect method, remarkable for the strange circum- 

 stance that the refraction of the rays was deduced from 

 experiments in which no refraction took place. Thus 

 our colleague found more than one incredulous reader. 

 The singularity of the discovery, perhaps, demanded 

 some hesitation : perhaps also in the eyes of some per- 

 sons, it had the fault, like the law of Huyghens, of being 

 the fruit of an hypothesis. However it may have been, 

 Fresnel met the difficulty boldly. By showing that in a 

 parallelepiped of topaz, formed of two prisms of the same 



ished ; then from the extremity t of the incident ray i as if produced 

 to meet this sphere, drawing tangent planes to the sphere and spheroid 

 respectively, the points of contact will give the position of the ordinary 

 and extraordinary rays o and e. See Peacock's Life of Young, p. 373. 

 Translator, 



