OPTICAL PHENOMENA OF CRYSTALS. 231 



into two rays of different refractive indices, or at least 

 that the thickness of crystal which is requisite to pro- 

 duce this effect is insensible to observation. 



When nitre is made to crystallize in gum, it often 

 shoots into prismatic crystals, which are very interesting 

 objects, the more so, that they are of a permanent nature, 

 and not liable to spontaneous change. When examined 

 by polarized light, these prisms, in one position of their 

 axis, frequently disappear completely. This arises from 

 the refractive power of the gum being equal to that of 

 one of the two rays in the crystal. Reverse the polari- 

 zation of the ray, and the crystal appears, as it were, to 

 start into existence, acquiring great strength and black- 

 ness of outline, and, not unfrequently, entire opacity. 

 Again, when the sulphate of lime is interposed, this 

 opacity disappears, and the crystal becomes brightly 

 coloured. Since it is probable that many better methods 

 may be found of obtaining this kind of crystal than have 

 hitherto presented themselves, I have hopes that it will 

 be possible to obtain large and permanent artificial 

 crystals, which may possess the advantages of the tour- 

 maline without the inconvenience resulting from its dark 

 colour. 



