102 Sir David Brewster on a Modification of the Doubly 



presented to me, were very indefinite in their character, and 

 very imperfectly developed ; but after a diligent examination 

 of nearly 900 specimens of topaz, 1 succeeded in obtaining the 

 most satisfactory 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 commu- 

 nicated 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 me- 

 dium, whether it be temporary or permanent, may be exhibited 

 to the eye in two ways ; either by the property of the com- 

 pressed parts in depolarizing light, or in the unequal refraction 

 of common light produced by a varying density, and conse- 

 quently a varying refractive power. In ihejirst of these cases, 

 the depolarizing action is displayed in the production of four 

 quadrants of light, separated by the radii of a black rectan- 

 gular cross, similar to the central portion, or the tints of the 

 first order, in the uniaxal system of polarized rings ; and, in 

 the second case, the inequality of refractive density is shown 

 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 depolarized light, 

 shown at A, B, C, D in Plate I. fig. 1, first presented them- 

 selves to me, 1 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 intersections of the arms of 

 the black cross, from which the compressing force had ema- 

 nated. One of these cavities is shown at E, fig. 2. It is of 

 a quadrangular form, like the section of a rhbmboidal prism, 

 sometimes elongated, and sometimes of a slightly irregular 

 shape. When perfectly regular, these cavities are between 

 the SOOOdth and the 4000dth of an inch in diameter. They 

 are always dark, as if the elastic substance which they con- 

 tained had collapsed into an opake powder ; and I have met 



