IV. Further observations on the optical phenomena of crystals

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; 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 occasionally; 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.