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II. — On the Modification of the Doubly Refracting and Physical Structure of 

 Topaz, by Mastic Forces emanating from Minute Cavities. By Sib David 

 Bkewstek, K.H., D. C. L., F. E. S., and V. P. R. S., Edin. 



(Read 20tli 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 ; hut 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 in a 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 first 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 he 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 ici the centre of the luminous quadrants, or at the 



