the Disintegrated Surfaces of Crystals. 19 



lous figure with a crescent at each end, and an elliptical space in 

 the centre, from which the image of the candle or luminous 

 point has wholly disappeared. Hence it appears that the whole 

 of the original surface of the flat summit of the crystal has been 

 removed by the action of the solvent, an effect which may be 

 imitated, as we shall presently see, in artificial crystals. The 

 nebulous expansion of which we have been treating has some- 

 times rectilineal branches at its extremities, and is sometimes 

 filled up in the middle, where the image of the candle is distinctly 

 seen. In other specimens, this nebulous portion is the only part 

 that is visible. The angular magnitude of the figure varies 

 greatly in different specimens, and also its distinctness and con- 

 tinuity. When the elementary facets are large, the outline of 

 the figure is marked by separateimagesof the candle; and when 

 these facets are very small, the luminous tracery is soft and 

 nebulous, and sometimes shading off into coloured tints, like the 

 fringes produced by the interference of common or polarized light. 



The optical figures produced by the faces of the pyramid are 

 less distinct and beautiful, but not less remarkable, than those 

 which we have been describing. Upon faces inclined about 145° 

 to the summit plane, and whtdi seem to be those marked s by 

 Hauy*, the strange figure shown at A, fig. 3, is seen ; on the 

 adjoining face the same figure reversed is seen as shown 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. 1 have observed 

 other figures on faces differently inclined 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 disintegration on their surfaces. 



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

 Shaionce town, Illinois, U.S., sent to me by Professor Silliman, 

 I have observed a figure consisting 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 f. 



In a crystal of Hornblende, the four summit planes at each end 

 of the prism give the figure of a small luminous circle, as shown 

 in fig. 4, the central image being wholly obliterated. On the 

 faces of the prism, which are not those of cleavage, the figure is 

 a luminous rhomboid, as shown in fig. 5, with a nebulous image 

 at each angle, and one in the centre, the shorter axis of the 

 rhomboid coinciding with the axis of the prism. In some speci- 



* Plate 44, fig. 1, &c., first edition. 



t These three radiations are ah, ac, be in fig. 12. See foot-note on p. 28. 



C2 



