26 Sir David Brewster on the Optical Figures produced by 



strange branching figure shown in fig. 27, where abc forms the 

 brightest portion. I obtained the same figure with another 

 crystal, but the parts xij were wanting, and b and c were con- 

 tinued through a to m and n. The side a was directed to the 

 obtuse angle of the rhomb. With another crystal, in which the 

 artificial face was inclined 104° instead of 71°, the figure shown 

 in PL V. fig. 28 was produced. 



My next experiments were made with sulphate of potash, a 

 crystal which belongs to the rhombohedral system. By the 

 shghtest action of water upon the flat summit of a hexagonal 

 prism it produced six luminous images, symmetrically arranged 

 round the central image, each image being opposite a side of the 

 hexagon. All these images were connected with the central 

 image by a halo of fainter light. The faces of the hexagonal 

 prism produce the figure shown in fig. 29, the line AB being 

 coincident with the axis of the prism. By continuing the action, 

 the branches C, D vanished, and the figure appeared as in fig. 30, 

 the images being connected with a haze of light. 



A more remarkable eficct was produced with the faces of the 

 truncated pyramid. Three of the six faces produced the eficct 

 shown in fig. 31, while the other three alternate faces produced 

 the same figures, but without the wings E, E. Sometimes two 

 images are seen below B. 



My attention was now directed to the system of ciystallization 

 in which the base of the primitive crystal is a square. Having 

 immersed a fine crystal of Fa7'oe apophyllite in dilute nitric acid, 

 the summits of the prism were alone acted upon. They produced 

 a figure With, four rectangular radiations directed to the angles 

 of the summit, and four much shorter ones pointing to the sides 

 of the square summit. The four large rays appeared first con- 

 nected with a luminous web, and the four small ones were sub- 

 sequently developed. 



The very same figure, but with some modifications, was pro- 

 duced by the action of water upon the summit of the square 

 prism of sulphate of potash and copper. The small radiations 

 were produced last, as in apophyllite ; but what is remarkable, 

 they are directed to the angles, and not to the sides of the square 

 face. The extreme solubility of this salt renders it difficult to 

 develope the figure distinctly. 



From another crystal of the same class, superacetate of copper 

 and lime, I obtained the beautiful figure shown in fig. 32, where 

 the eight radiations are of equal length, and the images at their 

 extremities connected by beautiful curves of light concave 

 outwards. 



I have made a great number of experiments with crystals 

 belonging to the prismatic system, such as sulphate of magnesia, 



