XV. On the Effects of Compression and Dilatation in altering 
the Polarising Structure of Doubly Refracting Crystals. 
By Davin Brewster, LL. D. F. B.S. Lon. & Enry. 
(Read November 17. 1816.) 
Ta 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 
Vou, VIII. P. II. Nn applied 
