34 
MR. TALBOT ON THE OPTICAL PHENOMENA OP CRYSTALS. 
the solution put on a glass plate yields very good analytic crystals. These have a 
branched or plumose appearance, and assume beautiful colours in polarized light 
when a plate of sulphate of lime is placed beneath them. The microscope shows the 
colour to reside principally in the outline, but to the naked eye the whole film appears 
coloured. As these films may be obtained of large size, the phenomenon can be well 
seen by the unassisted eye. 
A very interesting experiment, and one which throws much light upon the cause of 
these appearances, is to transmit a beam of polarized light very obliquely through a 
small prism of nitre immersed in gum, and viewed with the microscope. Its outline 
then generally exhibits two colours instead of one : for while the edge of the prism 
which is on that side from whence the ray of light comes is, for instance, of a red 
colour, the opposite edge will appear green. Reverse the polarization of the light, 
and these colours are exchanged one for the other. This observation enables us to 
explain the origin of the phenomenon in a satisfactory manner, and to show why it 
only occurs in crystals possessing strong double refraction, like nitre, in which the 
refractive indices of the two rays are materially different. 
When a ray of common light is incident upon such a crystal, and therefore divides 
itself into two rays oppositely polarized, both rays are transmitted through the cen- 
tral parts of the crystal, which are bounded by parallel planes, or by planes approach- 
ing to parallelism. But when the bounding planes of the crystal are much inclined 
to each other, and therefore refract the light in the manner of a prism, the refractive 
indices of the rays may differ so much, that while one of them passes freely through 
such a prism, the other cannot pass at all, but suffers total internal reflexion, and is 
thereby dispersed ; just as if the prism had a larger refracting angle with respect to 
that ray than to the other. Therefore if two oppositely polarized rays are presented 
successively to such a crystal, as in our experiment, one of them will be transmitted, 
and the other not. That this is the true explanation appears from this, that when the 
oblique planes are well formed and clearly defined by the microscope, the colour also 
is accurately limited by the same boundary : so that while this part analyses the tints 
of a plate of sulphate of lime, the rest of the crystal is inactive. 
It may be inferred by analogy, that the same cause produces the analysing power 
of striated or fibrous surfaces, and of those in which the striae are too minute to be 
discernible (as in No. 1. supra, page 32) : for it is not the property of all crystals with 
striated surfaces to have the analytic power, but only of such as are doubly refrac- 
tive in a high degree. 
I have said that the capillary crystals (No. 3.) possess the analytic property, 
although their diameter is often evanescent even with a microscope. An important 
inference may be drawn from this, viz. that a ray of light, immediately on entering 
one of these crystals, subdivides itself into two rays of different refractive indices, or 
at least that the thickness of crystal which is requisite to produce this effect is insen- 
sible to observation. 
