172 SIR DAVID BREWSTER ON THE OPTICAL FIGURES PRODUCED BY 



A, B, E, were connected with C by the radiations, and in one case, where a 

 weaker acid was used, E was elevated farther above C, and a horizontal band of 

 light passed below C to the sides AD, BD. 



When strong vinegar was used as the solvent, I obtained the figure shewn in 

 Fig. 23, the letters having the same indications as in the preceding figures. 



I now proceeded to apply the solvents to the summit planes of the prism. 

 My first experiment was made on an artificial face, perpendicular to the axis. By 

 the action of vinegar it gave the figure shewn in Fig. 24, which consists of three 

 radiations, inclined 120° to each other, having its centre sometimes dai-k, and 

 sometimes occupied with a small image. The rudiments of other tliree radia- 

 tions, inclined 60° to the former, are distinctly visible, and beside a luminous 

 circle circumscribing the whole, there are three non-concentric circular arches, 

 similar to those seen in fluor-spar. 



The very same figure, with the exception of the circle and the circular arches, 

 was obtained from the action of dilute muriatic acid on the natural faces of the 

 cJiaux ca7'bonatee basee of Hauy. 



Having ground and repolished the artificial summit which exhibited Fig. 24, 

 I exposed it to the action of dilute muriatic acid, when I was surprised to see it 

 produce the strange figure shewn in Fig. 25. Although the symmetry of the 

 figure is hostile to the idea that its shape might have been partly the effect oi* 

 accident, yet I found it unaltered by repolishing, and again disintegrating the 

 surface, and what is still more decisive, I obtained the very same effect twice from 

 another crystal of calcareous spar.* 



By placing the crystal which gave this remarkable figure in a stronger acid 

 solution, it gave on both its faces the figm-e in Fig. 26, the light of which is 

 strongest in the circular arches. By continuing the action of the same acid, the 

 three inclosed radiations disappear entirely, and what is still more singular, they 

 reappeared by a farther continuance of the action. The action being prolonged, 

 they again disappeared, the circular arches grew wider and more confused, tOl 

 they filled up the space which they at first inclosed. 



Another crystal of spar exhibited the very same series of successive changes 

 which I have now described. 



I now reground and polished the faces of both these specimens. When they 

 ,vere plunged into strong dUute acid, their disintegrated surfaces produced no 

 figure, but by increasing the strength of the solution the figures were developed 

 as formerly. 



In order to observe the effect produced upon faces that were not coincident 

 either with the primary or secondary faces of the crystals, I ground down one of 



* The brightest part of the figure was ab, the part above a being faint. 



