ON THE OPTICAL PHENOMENA OF CRYSTALS. 217 



optical figures which Brewster has discovered in spheres 

 of glass whose density was rendered variable by heating 

 them. 



He says* that " if we take a cold sphere of glass and 

 immerse it in a trough of hot oil, placed in a polarizing 

 apparatus, we shall observe a black cross with four sectors 

 of polarized light. If the sphere is turned round, it will 

 exhibit in every position the very same figure. If we 

 now suppose the trough to be filled with such spheres, 

 they will exhibit the same phenomena in whatever direc- 

 tion the polarized light is transmitted through them, and 

 even if they were in a state of motion. A fluid com- 

 posed of such spherical particles would exhibit the same 

 polarizing structure in every possible direction, and even 

 if it were in a rapid state of gyration. If the particles 

 possessed the structure that produces circular polariza- 

 tion, the fluid would develop the phenomena exhibited 

 by oil of turpentine, &c." 



And again-]-, " The structure of the particles of a cir- 

 cularly polarizing fluid must be exactly the same along 

 every one of its diameters ; that is, the structure must 

 be symmetrical round the centre of the particle, or 

 analogous to that which takes place in common polariza- 

 tion when a sphere of glass has its density regularly 

 increasing or regularly diminishing towards its centre." 



I have quoted these remarkable passages at length, 

 because it appears to me that what is there advanced 



* Library of Useful Knowledge, art, " Polarization of Light," p. 51, 

 t Ibid. p. 45. 



