the Disintegrated Surfaces of Crystals. 28 



the acid, or the duration of its action, one of the most interesting 

 is the one represented in fig. 16, where the three principal radia- 

 tions are inclosed in a luminous equilateral triangle, having a 

 bright image at each of its angular points. If we grind and 

 polish the opposite surface of the octohedron, so as to have a 

 parallel plate, we shall see fig. 16 much more brilliantly by- 

 transmitted light*. If we now expose this second surface to 

 the action of the acid, we shall see the optical figure shown in 

 fig. 17, which is fig. 16 inverted. The cause of this inversion 

 is, that this second face is parallel to a face in the opposite 

 pyramid of the octohedron, whose apex lies in an opposite direc- 

 tion to that of the face which gives fig. 16. If we now look 

 through the two faces that have been acted upon by the acid, we 

 shall see the beautiful luminous figure shown in fig. 18, each 

 image produced by the one surface being converted into an 

 optical figure by the second. When the figure produced by the 

 first surface of the plate of spar has its simplest form of three 

 radiations, the multiplied figure seen by transmission contains 

 only the twelve bright images and the central image of fig. 18; 

 but when it exhibits the more compound form of fig. 12 or 13, 

 the transmitted figure becomes exceedingly complex. It is 

 obvious that the figure shown in fig. 14 will not be altered by 

 transmission through two surfaces. Its brilliancy, however, and 

 distinctness will be increased. 



In some specimens I have observed three beautiful luminous 

 arches, mn, mo, no, as shown in fig. 12. 



Upon the face of a cube of fluor-spar, which had been ground 

 and smoothed, but not polished, before it was acted upon by 

 dilute muriatic acid, I observed the appearance in fig. 19. The 

 original image had entirely disappeared from the centre of the 

 rounded square of light, and the interior of the square was filled 

 up with a faint nebulous light of uniform intensity. The eight 

 round images were equidistant and equally bright, and the peri- 

 meter of the square was brightest at its angles and the middle 

 points of its sides. 



After the specimen had been exposed for some time to the 

 action of boiling muriatic acid, the face which had given fig. 19 

 now exhibited the remarkable phsenomena shown in fig. 20. 

 The nebulosity had almost disappeared from the interior of the 

 square, and collected, as it were, in its centre. The brightest 

 parts of the figure were the curved masses at the angles, the 

 middle parts of the sides of the figure being exceedingly faint. 



From the tessular I proceeded to the rhombohedral system of 

 crystallization, and I employed calcareous spar and sulphate of 

 potash in the inquiry. 



* This is the case with all the optical figures previously described. 



