404 HISTORY OF OPTICS. 



fied phenomena which Huyghens himself had not 

 observed. From this time, then, the truth of the 

 Huyghenian law was universally allowed, and soon 

 afterwards, the theory by which it had been sug- 

 gested was generally received. 



The property of double refraction had been 

 first studied only in Iceland spar, in which it is 

 very obvious. The same property belongs, though 

 less conspicuously, to many other kinds of crystals. 

 Huyghens had noticed the same fact in rock- 

 crystal 5 ; and Malus found it to belong to a large 

 list of bodies besides ; for instance, arragonite, sul- 

 phate of lime, of baryta, of strontia, of iron ; 

 carbonate of lead ; zircon, corundum, cymophane, 

 emerald, euclase, felspar, mesotype, peridote, sul- 

 phur, and mellite. Attempts were made, with im- 

 perfect success, to reduce all these to the law which 

 had been established for Iceland spar. In the first 

 instance, Malus took for granted that the extra- 

 ordinary refraction depended always upon an oblate 

 spheroid ; but M. Biot 6 pointed out a distinction 

 between two classes of crystals in which this spheroid 

 was oblong and oblate respectively, and these he 

 called attractive and repulsive crystals. With this 

 correction, the law could be extended to a con- 

 siderable number of cases ; but it was afterwards 

 proved by Sir D. Brewster's discoveries, that even 

 in this form, it belonged only to substances of which 



5 Traile de La Lumicre, ch. v. Art. 20. 

 9 Biot, Traite de Phys. iii. 330. 



