I 70 SIR DAVID BREWSTER ON THE OPTICAL FIGURES PRODUCED BY 



images appear also at /, m, n, distant 120° from each other, and 60° from the 

 principal radiations. 



When the faces of the cube formed by planes replacing the angles of the 

 octohedron are acted upon by the acid, the beautiful figm-e shewn in Fig. 14. is 

 produced, the half moons at the four angles being more distinctly brought out in 

 some cases than in others. 



The mutual connection of these figures will be seen in P'ig. 15, where the tri- 

 angles represent the faces of the octohedral pyi-amid imfolded as it Avere, and the 

 quadrangular figure the square base of the pyramid. 



Among the vai-iations of figure produced by the strength of the acid, or the 

 duration of its action, one of the most interesting is the one represented in Fig. 16, 

 where the three principal radiations are inclosed in a luminous equilateral trian- 

 o-le, 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 bx-Uliantly by transmitted light.* If we now expose this 

 second smface to the action of the acid, we shall see the optical figure shewn in 

 Fig. 1 7, which is Fig. 16. inverted. The cause of this inversion is, that this second 

 face is parallel to a face in the opposite pyi-amid of the octohedron, Avhose apex 

 lies in an opposite direction to that of the face which gives Fig. 16. If we now 

 look thi-ouo-h the two faces that have been acted upon by the acid, we shall see 

 the beautiful lummous figure shewn 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 fii"st surface of the plate of spar has its sunplest form of three 

 radiations, the multipUed figure seen by transmission contains only the twelve 

 brio-ht 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 she^vn 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, m n, 

 mo, no, as shewn in Fig. 12. 



Upon the face of a cube of fluor-spai-, 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 entu-ely disappeared from the cen- 

 tre of the rounded square of light, and the interior of the cube was filled up with a 

 faint nebulous light of unifonn intensity. The eight round images were equidis- 

 tant and equally bright, and the perimeter of the square was brightest at its angles 

 and the middle points of its sides. 



* This is the ca.se witli all the optical figures previously described. 



