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IV. Further Observations on the Optical Phenomena of Crystals. 
By H. F. Talbot, Esq. F.R.S. 
Received October 26, — Read December 15, 1836. 
§ 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 ; 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. 
1. The circles are much larger, attaining the diameter of -p-th of an inch ; whereas 
those first observed did not exceed -^-^dth of an inch in diameter. 
2. They are flat, whereas the former ones were convex, and frequently I believe 
of a spherical form. 
3. In consequence of which probably, they are seen to exhibit no coloured rings, 
but only a cross. 
4. The cross is brightly coloured, red, green, blue, &c. upon a white ground (the 
polarizers being supposed to be parallel to each other). This has a beautiful appear- 
ance, especially when several circles seen at once have crosses of different colours. 
5. When the polarizers are placed at right angles, the phenomenon complementary 
to the above is seen. For instance, the circle which presented a red cross upon a 
white ground now presents a black cross upon a green ground. 
6. In an intermediate position of the polarizers, the circle just mentioned presents 
a red cross alternating with a green one, thus dividing the circle into eight sectors of 
coloured light. 
Other circles present other colours, but they all follow the same analogy, and the 
crosses upon all the crystals are in a parallel position. 
7. These crystals last longer than the former ones. I have found some of these re- 
tain their structure for two months ; the former kind seldom last in perfection more 
than a day. 
8. It sometimes happens that their circumference is imperfect, and presents a 
