Igg SIK DAVID BREWSTER ON THE OPTICAL FIGURES PRODUCED BY 



cabinets, owing to their being in general bad specimens, I have not been able 

 to pursue this branch of the subject any farther ; but I have uo doubt that if I 

 had such a copious supply of other minerals as I had of topaz, I shotdd be able 

 to find among them specimens of equal interest. 



II. We come now to the second and the principal branch of the subject, — ^to 

 describe the optical figures produced by the action of water, acids, and other sol- 

 vents, upon the surfaces of perfect crystals, both natural and artificial. 



The crystals which I have found to be best adapted for exhibiting the action 

 of solvents in producing optical figures by reflection, are Alum, Flzior-spar, and 

 Calcareous Spa?'. 



If we take a fine crystal of Alum, and look at the image of a candle reflected 

 as perpendiculai'ly as possible from one of the faces of the octohedron, it will ap- 

 pear perfectly distinct, and without any Imninous appendages. If we now im- 

 merse it for an instant in water, and dry it quickly with a soft cloth, the reflected 

 image wUl send out thi-ee luminous radiations, as she^vn in Fig. 7. By a second 

 immersion in the water, thi-ee small unages of the candle will be developed at 1. 

 2, 3 ; and by a little farther action of the solvent, these images connect them- 

 selves with the central image S, by the radial lines 1 S, 2 S, 3 S, inchued 110° to 

 each other, and 30° to the principal radiations from S. By continuing the action 

 other three images stail; up at 4, 5, 6, but apparently without any radial connec- 

 tion with S. The principal radiations a S, ft S, c S begin at this period to grow 

 faint between 4 and 1, 5 and 2, and 6 and 3. Another immersion of the ci-ystal 

 developes the images 7, 8, 9 ; and by continuing the action, the images 1, 2, 3 

 become the brightest, and the branches A, B, C become more like images at m, n, o. 

 The central image S has now transfeiTed almost all its light to the new unages, 

 and another immersion wiU make it disappear altogether, leaving the central part 

 of the figm-e perfectly dai'k, as in Fig. 8. 



It is now obvious, that by repeated actions of the solvent we have removed 

 the whole of the original surface of the crystal by which the central image S was 

 formed, and have replaced it by a gi'eat number of facets, which reflect, in conse- 

 quence of theu" various incUnations, the difierent portions of the geometrical image 

 shewn in Fig. 8. If we cany the process of solution farther, the figure Avill un- 

 dergo successive changes, becoming lai'ger and more discontinuous in its outHne. 

 The beauty and regidar development of these phenomena, depend in some 

 measm-e on the perfection of the original surface of the crystal, and greatly on the 

 uniform temperatm-e of the water, and the shortness of the period during which 

 the crystal is immersed in it. The successive development of the figure may be 

 pretty well seen upon an artificial sm-face of the octohedron of alum, provided it 

 is nearly parallel to the original surface. When the inclination of the artificial 



