32 DISCOURSE ON THE STUDY 



plane and extraordinarily refracted, while the other 

 followed the ordinary rule. Accordingly this was 

 supposed to be the case ; and not only so, but from 

 some trials and measurements purposely made by a 

 philosopher of great eminence, it was considered to 

 be a fact sufficiently established by experiment. 



(23.) Perhaps we might have remained long un- 

 der this impression, for the measurements are deli- 

 cate, and the subject very difficult. But it has lately 

 been demonstrated by an eminent French philosopher 

 and mathematician, M. Fresnel, that, granting certain 

 principles or postulates, all the phenomena of double 

 refraction, including perhaps the greatest variety of 

 facts that have ever yet been arranged under one 

 general head, may be satisfactorily explained and 

 deduced from them by strict mathematical calcula- 

 tion ; and that, when applied to the cases first men- 

 tioned, these principles give a satisfactory account 

 of the want of the extraordinary image ; that when 

 applied to such cases as those of rock-crystal or Ice- 

 land spar, they also give a correct account of both 

 the images, and agree in their conclusions with the 

 rules before ascertained for them : but so far from 

 coinciding with that part of the previous statement, 

 which would make these conclusions extend to all 

 crystallised substances, M. Fresnel's principles lead 

 to a conclusion quite opposite, and point to a fact 

 which had never been observed, viz. that in by far 

 the greater number of crystallized substances which 

 possess the property of double refraction, -neither of 

 the images follows the ordinary law, but both un- 

 dergo a deviation from their original plane. Now 

 this had never been observed to be the case in any 



