30 
MR. TALBOT ON THE OPTICAL PHENOMENA OF CRYSTALS. 
notched or jagged outline. These have a very beautiful appearance, and have been 
almost universally compared by those who have looked at them to highly coloured 
flowers with four petals ; the cross upon them being so dark as to have the appear- 
ance of being a division between the petals. 
All the circles, when viewed by common light, appear transparent, white, and very 
uniform. If they are composed of acicular crystals diverging from a point, these 
latter must be exceedingly slender and numerous, and in perfect optical contact, 
since a high magnifying power does not render them separately visible. 
§ 2 . 
With respect to the chemical nature of these crystals, it appears to me evident that 
they consist of boracic acid. They are obtained by dissolving borax in phosphoric 
acid ; and it may be inferred that this latter substance unites with the alkali and 
isolates the boracic acid. In order to see if this supposition were correct, I dissolved 
boracic acid in alcohol, and I found that a drop of this solution evaporated on a plate 
of glass frequently yielded an abundant crop of the crystalline spherules. But these 
are generally exceedingly small, requiring a high power to display in them the cross 
and four sectors of light, and they speedily grow opake ; for which reason they are 
not so well suited for observation as those prepared by the former method. They 
establish the fact, however, that this mode of crystallizing is a property of the boracic 
acid. It is highly improbable that it should be peculiar to that substance, but I have 
not yet met with it in any other. 
§ 3. Explanation of some of the Optical appearances. 
1. When any doubly refracting crystal is examined with the polarizing microscope, 
(the polarizers being transverse to each other, and the field of view consequently 
dark,) if it be turned round in one plane, it is seen to grow four times luminous and 
four times dark in the course of one revolution. This I have found to be universally 
the case with all the substances which I have tried, and it also is in accordance with 
theory. 
2. In the case of an acicular crystal, one of the optical axes always coincided with 
the axis of figure, or length of the crystal ; so that if a crystal of this sort appears 
unilluminated, all the others that are either parallel to it or perpendicular to it are 
likewise dark. 
3. It results from the above that a circle composed of acicular crystals diverging 
from a point, must present the appearance of a black cross, and that the crosses on 
all the circles will be parallel. 
4. With respect to the rings of colour, they are a consequence of the variable 
thickness of the crystalline circle at different distances from its centre. Their being 
visible, and indeed very conspicuous, upon a body of such small diameter, arises from 
the very energetic action of boracic acid upon polarized light. 
