THE DISINTEGRATED SURFACES OF CRYSTALS. ] (37 



joining face the same figure reversed is seen as shewn at B ; on the next face is 

 seen the figure C, the same as A ; and on the next face again the figure D, the 

 same as B. I have ohserved other figures on faces differently inchned to the axis, 

 but they are not of sufficient distinctness to merit delineation. 



Optical figures analogous to those seen in topaz, may be observed in various 

 other minerals ; but it is very difficult to find specimens that have undergone dis- 

 integration on their surfaces. 



On the cubical faces of a specimen of White Fluor-spar from Shaionce town, 

 lUinois, U. S., sent to me by Professor Silliman, I have observed a figure consist- 

 ing of four radiations, inclined 90° to each other, and having the bright central 

 image entirely obliterated. On the octohedral surfaces of the common fluor-spar, 

 the figure consists of three radiations, inclined 120* to each other. 



In a crystal of Ho^^nhlende, the four summit planes at each end of the prism 

 give the figure of a small luminous circle, as shewn in Fig. 4, the central image 

 being wholly obliterated. On the faces of the prism, which are not those of 

 cleavage, the figm-e is a luminous rhomboid, as shewn in Fig. 5, with a nebulous 

 image at each angle, and one in the centre, the shorter axis of the rhomboid co- 

 inciding with the axis of the prism. In some specimens the luminous lines uni- 

 ting the four images at the angles are not developed. 



In a specimen of Axinite, I observed the remarkable geometrical figure shewn 

 in Fig. 6. It consisted of two images a, b, joined by a line of light, and each of 

 them sending out, in opposite but parallel directions, luminous rectilineal branches 

 ac, hd. The line « 5 is perpendicular to the edges of the prism, and ac,hd pa- 

 rallel 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 re- 

 flection 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 gives six luminous radii, in- 

 clined 60° to each other ; but each alternate image is stronger than the one adja- 

 cent to it. 



On the rhomboidal faces of the dodecahedron. Garnet gives an optical figure 

 like a St Andrew's cross, the line bisecting the arms of the cross being perpendi- 

 cular to the longer diagonal of the rhomboidal face. 



The natural faces of a fine octohedral Diamond gave three luminous radia- 

 tions, inclined 120° to each other ; and the same figure was exhibited by the faces 

 of a rough pyramid of Amethyst, and by some of the cleavage planes of Oligist 

 Iron-ore. 



As minerals with disintegrated surfaces are not to be found in mineralogical 



