IQQ SIR DAVID BREWSTER ON THE OPTICAL FIGURES PRODUCED BY 



they were rough and irregular, as if they had been disintegrated by a solvent ; and 

 I observed the very same effect on the flat summits and pyramidal faces, but never 

 on the faces of the prism. As it was impracticable to apply the goniometer to the 

 mensuration of the angles of the minute facets which the microscope rendered vi- 

 sible on these disintegrated surfaces, I thought of obtaining a general idea of their 

 position by examining the manner in which they arranged the reflected images of 

 a luminous point placed at a distance. Upon making this experiment, I was sur- 

 prised to see a beautiful optical flgure, consisting ofthe most elegant curves of con- 

 trary flexure, studded with tufts of light, and arranged with the most perfect sym- 

 metry round the central image of the luminous point which is formed by those por- 

 tions ofthe summit ofthe crystal which had escaped from the action ofthe solvent. 

 This remarkable arrangement of the reflected light is shewn in Plate X, Fig. 1. 

 Avhere it consists of three curves of contrary flexure of the general form of lemnis- 

 cates, having at the extremities of their greater axis two semicircular tufts of light, 

 and at the extremities of their lesser axis two triangular tufts of light. These 

 figures undergo considerable changes on different specimens, depending, as will 

 afterwards be seen, either on the time during which the solvent has acted upon 

 it, or upon its dissolving power : — but they never deviate from the general type, 

 and in the most imperfect and rough specimens, of which I have examined more 

 than an hundred, it is easy to recognise the elements of the perfect figure. One 

 of these variations in the figure is shewn in Fig. 2, where the light of the inner 

 curve is diffused over a nebulous figure with a crescent at each end, and an ellip- 

 tical 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 sometimes 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 continuity. When the ele- 

 mentary facets are large, the outline of the figure is marked by separate images 

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

 and nebulous, and sometimes shading off into colom-ed tints, like the fringes pro- 

 duced by the interference of common or polarized hght. 



The optical figures produced by the faces of the pjramid 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 which seem to be those 

 marked s by Hauy,* the strange figure shewn at A, Fig. 3, is seen ; on the ad- 



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



