168 SIR 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 no doubt that if I 

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

 to find among them specimens of equal interest. 



11. 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, Fluor-spar, and 

 Calcareous Spar. 



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

 as perpendicularly as possible from one of the faces of the octohedron, it will ap- 

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

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

 image will send out three luminous radiations, as shewn in Fig. 7. By a second 

 immersion in the water, three small unages of the candle wiU 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, inchned 110° to 

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

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

 tion with S. The principal radiations a S, 6 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 crystal 

 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 transferred almost aU its light to the new images, 

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

 of the figure 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 great number of facets, which reflect, in conse- 

 quence of their various inclinations, the different portions of the geometrical image 

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

 dergo successive changes, becoming larger and more discontinuous in its outhne. 

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

 measure on the perfection of the original surface of the crystal, and greatly on the 

 uniform temperature of the water, and the shortness of the jjeriod during which 

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

 pretty weU 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 



