18 Sir David Brewster on the Optical Figures produced by 



to the proper subject of this paper, which is to describe the 

 optical tigurcs produced by the disintegrated surfaces of minerals 

 and artificial crystals. The disintegration by which these figures 

 are developed, is produced by three causes : — 



I. By the natural action of solvents on the mineral, either at 

 the time of its formation or at some subsequent period in the 

 bowels of the earth. 



II. By the action of acids and other solvents upon the surfaces 

 of perfect crystals ; and, 



III. By mechanical abrasion. 



I. The first examples of Natural disintegration which I met 

 with were in Brazil Topaz. In a great number of these topazes 

 I observed cavities filled with a white pulverulent substance, 

 which Berzelius, who analysed it at my request, found to be a 

 sort of marl, consisting of silex, alumina , lime, and water, and 

 which, as he remarks, would have formed a zeolite had it been 

 crystallized. Upon examining the sides of the cavities which 

 contained this substance, I found that they were rough and irre- 

 gular, as if they had been disintegrated by a solvent ; and I 

 observed the very same efiect on the flat summits and pyramidal 

 faces, but never on the faces of the prism. As it was impracti- 

 cable to apply the goniometer to the mensuration of the angles 

 of the minute facets which the microscope rendered visible 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 surprised to see 

 a beautiful optical figure, consisting of the most elegant curves 

 of contrary flexure, studded with tufts of light, and arranged 

 with the most perfect symmetry round the central image of the 

 luminous point which is formed by those portions of the summit 

 of the crystal which had escaped from the action of the solvent. 

 This remarkable arrangement of the reflected light is shown in 

 Plate I. fig. 1, where it consists of three curves of contrary flex- 

 ure of the general form of lemniscates, having at the extremities 

 of their greater axis two semicii'cular tufts of light, and at the 

 extremities of their lesser axis two triangular tufts of light. 

 Thesefigures undergo considerable changes ondifierent 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 a hundred, it is easy to recognise the elements of the per- 

 fect figure. One of these variations in the figure is shown in 

 fig. 2, where the light of the inner curve is difKised over a nebu- 



