194 | ' FRESNEL. 
During the long discussions which took place araong 
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- 
parallelopiped of topaz, formed of two prisms of the same 
ished; then from the extremity ¢ 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 ande. See Peacock’s Life of Young, p. 373.— 
Translator. 
