DOUBLE REFRACTION. 73 



II. Uniaxal crystals, or those which have one axis of 

 double refraction. 



III. Biaxal crystals, or those which have two such 

 axes. 



Sir David Brewster has established a connexion between 

 these diversities of optical character and the varieties of crys- 

 talline form. He has shown that all the crystals of the first 

 class, i. e. all single-refracting crystals, belong to the tessular 

 system of Mohs ; that all uniaxal crystals belong either to 

 the rhombohedral or to the pyramidal system ; and that crys- 

 tals of the third class, or biaxal crystals, belong to one or 

 other of the prismatic systems. 



These important relations bear, in a very close and defi- 

 nite manner, upon the proximate cause of double refraction. 

 It has been just mentioned, that the only crystals which do 

 not possess the property of double refraction are those belong- 

 ing to the tessular system, i. e. those whose fundamental form 

 is the cube. Now in this, and its derived forms, we can assign 

 three lines at right angles to one another, round which the 

 whole figure is symmetrical ; and we may, therefore, reason- 

 ably conclude that the density and elasticity of the crystal 

 is the same in each of these directions, and consequently the 

 same throughout. Again, crystals with one axis of double 

 refraction belong either to the rhombohedral, ortotliepramidal 

 system, systems whose fundamental forms are the rhombo- 

 hedron, and the straight pyramid. In each of these forms there 

 is one axis of figure, or one line round which the whole is 

 symmetrical : and we may, therefore, assume that the density 

 of the crystal is either greater or less in this direction than in 

 others, while it is equal in all directions at right angles to it. 

 The axis of form is, in this case, the axis of double refrac- 

 tion. Finally, in the oblique pyramid, which is the funda- 

 mental form of the prismatic systems, there is no one line, or 

 axis of figure, round which the whole is symmetrical ; and it 



