XV. On the Effects of Compression and Dilatation in altering 

 the Polarising Structure of Doubly Refracting Crystals. 

 By David Brewster, LL. D. F. R. S. Lond. & Edin. 



(Read November 17. 1816.) 



In a paper which I had the honour of submitting to the So- 

 ciety, at the end of last session, I gave a detailed account of 

 the effects of mechanical compression and dilatation, in com- 

 municating to glass and other uncrystallised bodies, all the 

 properties of doubly refracting crystals* I had at that time at- 

 tempted, without success, to alter the polarising structure of 

 doubly refracting crystals, although I applied the force of 

 powerful screws to Topaz, Rock-crystal, and Calcareous spar. 

 All the specimens which I employed were crushed to pieces 

 by the pressure, but exhibited no traces of the coloured rings 

 when exposed to polarised light. 



The cause of the failure of these experiments did not occur 

 to me, till I was engaged in examining the phenomena pro- 

 duced in the direction of the resultant axes of regular crystals. 

 I then saw, that the pressure formerly applied, had actual- 

 ly developed a new polarising force, but that it had been 



Vol. VIII. P. II. N n applied 



