Il.—On the Modification of the Doubly Refracting and Physical Structure of 
Topaz, by Elastic Forces emanating from Minute Cavities. By Str Davin 
Brewster, K. ., D.C. L., F.R.S., and V. P. R. S., Edin. 
(Read 20th January 1845.) 
WHILE examining, in polarised light, the form and structure of the numerous 
crystals which I had discovered in the fluid cavities of Topaz, my attention was 
particularly called to certain optical phenomena exhibited in other parts of the 
specimen. These phenomena, when first presented to me, were very indefinite 
in their character, and very imperfectly developed ; but after a diligent examina- 
tion of nearly 900 specimens of topaz, I succeeded in obtaining the most satisfac- 
tory exhibition of them under various forms, and in various degrees of intensity. 
When an elastic force is propagated from a centre, in a soft and compressible 
medium, an increase of density is communicated to the surrounding mass,—of a 
temporary nature if the medium is a hard solid, like glass, but of a permanent 
nature if the medium is soft, and becomes indurated during the continuance of 
the compressing force. Both these effects may be exhibited experimentally, the 
first by a pressure upon glass, and the second by the action of an expanded bubble 
of air upon gum ina state advancing to induration. 
The physical change thus produced in the transparent medium, whether it 
be temporary or permanent, may be exhibited to the eye in two ways, either by 
the property of the compressed parts in depolarising light, or in the unequal re- 
fraction of common light produced by a varying density, and consequently a 
varying refractive power. In the jirst of these cases, the depolarising action is 
displayed in-the production of four quadrants of light, separated by the radii of a 
black rectangular cross, similar to the central portion, or the tints of the first 
order, in the uniaxal system of polarised rings; and, in the second case, the in- 
equality of refractive density is shewn by the mirage of a luminous point, in the 
form of concentric circles surrounding the centre of force, each circle marking 
successive actions of the central force. 
When the four luminous quadrants of depolarised light, shewn at A, B, C, D, 
in Plate, Fig. 1, first presented themselves to me, I had some difficulty in perceiving 
the seat of the force, by which I believed that they were produced. The centres, or 
intersections of the black cross, were either too deep beneath the surface of the 
topaz, or too much covered by fluid cavities, to be seen; but by removing the part 
of the crystal which contained these cavities, I succeeded in finding that, in every 
case there was a minute cavity in the centre of the luminous quadrants, or at the 
