219 



No. 2. 



Further Observations on the Optical Phenomena of Crystals. 

 By H. F. TALBOT, Esq. F.R.S.* 



Sect. I. — In my former paper on this subject I have 

 described the remarkable circular mode of crystallization 

 which sometimes occurs when borax crystallizes from a 

 solution in phosphoric acid. 



I have stated that when examined by the polarizing 

 microscope, a black cross and four sectors of light are 

 seen upon each crystal j and upon that kind which is 

 most easily and frequently obtained, there are seen in 

 addition one or more rings of vivid colour. Some 

 deviations, however, from this usual form occur occa- 

 sionally ; one of which, being extremely beautiful as a 

 microscopic object, deserves a separate mention. This 

 variety of crystalline circles differs from the one first 

 described in the following particulars. 



1. The circles are much larger, attaining the diameter 

 of one-tenth of an inch ; whereas those first observed did 

 not exceed one-two-hundredth of an inch in diameter. 



2. They are flat, whereas the former ones were convex, 

 and frequently, 1 believe, of a spherical form. 



* From Phil. Trans. Part I. for 1837. 



