20 Sir David Brewster on the Optical Figures produced by 



mens the luminous lines uniting the four images at the angles 

 arc not developed. 



In a specimen of Axinite, I observed the remarkable geome- 

 trical figure shown in fig. 6. It consisted of two images a, b, 

 joined by a line of light, and each of them sending out, in oppo- 

 site but parallel directions, luminous rectilineal branches ac, bd. 

 The line ab is perpendicular to the edges of the prism, and ac, 

 bd parallel to the sides of the reflecting face. On the opposite 

 side of the prism the figure is reversed. 



On the faces of the primitive cube of Boracite, the optical 

 figure seen by reflexion is a rectangular luminous cross, with a 

 central image, the radiations being perpendicular to the edges of 

 the square faces. Muriate of soda that had begun to deliquesce 

 in a humid atmosphere exhibits the same figure. 



The faces of the octohedron of oxidulated iron ^ives six lumi- 

 nous radii, inclined 60° to each other ; but each alternate image 

 is stronger than the one adjacent to it. 



On the rhomboidal faces of the dodecahedron. Game/ gives an 

 optical figure like a St. Andrew's cross, the line bisecting the 

 arms of the cross being perpendicular to the longer diagonal of 

 the rhomboidal face. 



The natural faces of a fine octohedral Diamond gave three lumi- 

 nous radiations, inclined 120° to each other ; and the same figure 

 was exhibited by the faces of a rough pyramid of Amethyst y and 

 by some of the cleavage planes of Oligist Iron-ore. 



As minerals with disintegrated surfaces are not to be found in 

 mineralogical cabinets, owing to their being in general bad spe- 

 cimens, I have not been able to pursue this branch of the subject 

 any further ; 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. 



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 solvents, upon the surfaces of 

 perfect crystals, both natural and artificial. 



The ciystals which I have found to be best adapted for exhi- 

 biting the action of solvents in producing optical figures by 

 reflexion, are Alum, Fluor-spar, and Calcareous Spar. 



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

 candle reflected as perpendicularly as possible from one of the 

 faces of the octohedron, it will appear perfectly distinct, and 

 without any luminous appendages. If we now immerse it for 

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

 reflected image will send out three luminous radiations, as shown 

 in fig. 7. By a second immersion in the water, three small 



