XIII. On the Position of the Axes of Optical Elasticity in Crystals 

 belonging to the Oblique-Prismatic System. By W. H. Miller, 

 M.A. F.R.S. Fellow and Tutor of St. John's College, and Pro- 

 fessor of Mineralogy in the University of Cambridge. 



[Read March 21, 1836.] 



In a Memoir printed in the 5th Volume of the Cambridge Trans- 

 actions it is stated, that in crystals belonging to the Oblique-Prismatic 

 System one of the three rectangular axes of optical elasticity was 

 always found to coincide with that crystallographic axis (Y, Y') which, 

 in crystals of this system, is perpendicular to the other two: but that 

 the positions of the other axes of optical elasticity (££', ££') had no 

 known relation to the form of the crystal. In some oblique-prismatic 

 crystals, however, it was found that one of the axes of optical elasticity 

 f £'» £T was a l so the axis of a principal zone. In the crystals which 

 I have examined since the publication of the paper already alluded to, 

 by the same method, this coincidence is found to occur less frequently. 

 Upon the whole, however, there seems to be no reason for supposing 

 it accidental in the instances (five or six out of twenty) in which it 

 has been observed ; but rather that it is a particular case of some 

 general law connecting the form and optical properties of crystals, in 

 the discovery of which it is hoped the observations here recorded may 

 be in some degree instrumental. 



The crystals selected for examination are taken principally from 

 among those which have been described by Mr Brooke in the Annals 

 of Philosophy for 1823 and 1824. The mutual inclination of two faces 

 is expressed by the angle between their normals, or the angular distance 

 of their "poles." An explanation of the notation in which the symbols 



Vol. VII. Part II. DD 



