290 On the Optical Phaenotnena of certain Crystals. 



by these little crystals, which are probably the minutest bodies 

 in which so complicated an optical structure has hitherto been 

 witnessed. I find that the smaller the circles are, the more 

 perfect is their form and the brighter their colours. 



These crystals, as I have already observed, probably con- 

 sist of spicula divei'ging from a point, but which are in the 

 closest possible contact, and in a state of complete mechani- 

 cal cohesion. It seems to follow as a consequence from such 

 a structure that their density must increase from their circum- 

 ference towards their centre. Now it is worthy of remark, 

 that Sir David Brewster has discovered very similar phaeno- 

 niena by polarized light in the crystalline lenses of certain 

 fishes, which are known by direct experiment to increase in 

 density towards the centre. Indeed the figure which he has 

 given of the lens of the codfish in the Philosophical Transac- 

 tions for 1816 (Plate XII. fig. 1.) is so like the appearance of 

 one of the crystals which I have described, that it might be 

 supposed to have been intended for a representation of it. 



Having pointed out this resemblance, I may also mention 

 another class of facts to which I think those I have de- 

 scribed possess a considerable analogy. I mean the optical 

 figures which Brewster has discovered in spheres of glass 

 whose density was rendered variable by heating them. 



He savs* that, " if we take a cold sphere of glass and im- 

 merse it in a trough of hot oil, placed in a polarizing appa- 

 ratus, we shall observe a black cross with four sectors of 'po- 

 larized 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 phaenomena in whatever direction the polarized light is 

 transmitted through them, and even if they were in a state of 

 motion. A fluid composed of such spherical particles would 

 exhibit the same polarizing structure in every possible direc- 

 tion, and even if it were in a state of rapid gyration. If the 

 particles possessed the structure that produces circular polari- 

 zation the fluid would develop the pheenomena exhibited by 

 oil of turpentine, &c." 



And againf, " The structure of the particles of a circularly 

 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 polarization when a sphere of glass has its 

 density regularly increasing or regularly diminishing towards 

 its centre." 



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

 t p. 45, ibid. 



