450 HISTORY OF OPTICS. 



such a difference, he deduces the existence of 

 spheroidal undulations. This suggestion appeared 

 in the Quarterly Review for November, 1809, in a 

 critique upon an attempt of Laplace to account for 

 the same phenomenon. Laplace had proposed to 

 reduce the double refraction of such crystals as 

 Iceland spar, to his favourite machinery of forces 

 which are sensible at small distances only. The 

 peculiar forces which produce the effect in this case, 

 he conceives to emanate from the crystallographic 

 axis ; so that the velocity of light within the crystal 

 will depend only on the situation of the ray with 

 respect to this axis. But the establishment of this 

 condition is, as Young observes, the main difficulty 

 of the problem. How are we to conceive refracting 

 forces, independent of the surface of the refracting 

 medium, and regulated only by a certain internal 

 line? Moreover, the law of force which Laplace 

 was obliged to assume, namely, that it varied as the 

 square of the sine of the angle which the ray made 

 with the axis, could hardly be reconciled with me- 

 chanical principles. In the critique just mentioned, 

 Young appears to feel that the undulatory theory, 

 and perhaps he himself, had not received justice at 

 the hands of men of science ; he complains of a per- 

 son so eminent in the world of science as Laplace 

 then was, employing his influence in propagating 

 errour, and disregarding the extraordinary confirm- 

 ations which the Huyghenian theory had recently 

 received. 



