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

 belonging to the Ohlique-Prismatic System. By W. H. Millek, 

 A.M. Fellow and Tutor of St John's College, and Professor of 

 Mineralogy. 



[Read Dec. 8, 1834.] 



1. Fresnel has proved that whatever be the regular arrangement 

 of the medium which by its elasticity produces the optical properties 

 of a crystal, there are always three directions at right angles to each 

 other, which may be considered as axes of optical elasticity. This 

 being understood, it is further already established, that crystals belong- 

 ing to the tesseral system have three equal axes of optical elasticity ; 

 that rhombohedral and pyramidal crystals have two axes of elasticity 

 equal to each other and perpendicular to the crystallographic axis, 

 which therefore is the third axis of elasticity and also an optic axis; 

 and that crystals belonging to the remaining systems have three unequal 

 axes of elasticity, and consequently two optic axes (that is, axes of 

 optical phenomena) making with each other angles which are bisected 

 by the axes of greatest and least elasticity. 



Sir David Brewster, who discovered the mutual dependence of the 

 forms and optical properties of crystals, has determined the angles be- 

 tween the optic axes of a great number of biaxal crystals; his obser- 

 vations, however, do not contain any data from which the positions of 

 the axes with respect to the faces of the crystals can be found. 



2. In the right prismatic system the axes of elasticity coincide (as 

 might have been expected) with the rectangular crystallographic axes. 

 In the oblique prismatic system, if the three axes be XX', YY', ZZ', 

 the crystallographic axis {YY'), which is perpendicular to the other 

 two {XX', ZZ'), is always one of the axes of elasticity. This, in 

 Gypsum, at the ordinary temperature of the air, and in many other 

 crj'stals, is the mean axis, or it is perpendicular to the optic axes; in 



