280 NOTES TO BOOK XV. 



Although we are thus led to distinguish the tessular and 

 the prismatic systems (using Mohs's terms) from the rhom- 

 bohedral and the square prismatic, we are not led to distin- 

 guish the latter two from each other ; inasmuch as they 

 have no optical difference of character. But this distinc- 

 tion is quite essential in crystallography ; for these two 

 systems have faces formed by laws as different as those of 

 the other two systems. 



Moreover, Weiss and Mohs not only divided crystal- 

 line forms into certain classes, but showed that by doing 

 this, the derivation of all the existing forms from the fun- 

 damental ones assumed a new aspect of simplicity and gene- 

 rality ; and this was the essential part of what they did. 



On the other hand, I do not think it is too much to 

 say, as I have said in the Philosophy of the Inductive Sci- 

 ences, B. vni. C. iii. Art. 3, that " Sir D. Brewster's optical 

 experiments must have led to a classification of crystals into 

 the above systems, or something nearly equivalent, even if 

 they had not been so arranged by attention to their forms." 



(N.) p. 246. Our knowledge with respect to the posi- 

 tions of the optical axes of oblique prismatic crystals is 

 still imperfect. It appears to be ascertained that, in 

 singly oblique crystals, one of the axes of optical elasticity 

 coincides with the rectangular crystallographic axis. In 

 doubly oblique crystals, one of the axes of optical elasticity 

 is, in many cases, coincident with the axis of a principal 

 zone. I believe no more determinate laws have been dis- 

 covered. 



(o.) p. 248. It will be seen by the account in the 

 text that Prof. Mitscherlielis merit in the great discovery 

 of Isomorphism is not at all narrowed by the previous con- 

 jectures of M. Fuchs. I am informed, moreover, that 



