32 
MR. TALBOT ON THE OPTICAL PHENOMENA OF CRYSTALS. 
colour which appear upon its crystals. These isochromatic lines, of which there are 
frequently many alternations, denote lines of equal thickness in the crystal. In the 
case of boracic acid, when anhydrous or nearly so, these lines are more crowded than 
in any other crystal that I have yet examined, insomuch that to exhibit them distinctly 
is as fine a test of the performance of a microscope as to resolve the more difficult 
lines on the scales of a butterfly’s wing, or any other of the known test-objects. And 
in many cases the microscope only indicates the existence of a still more delicate 
structure, which, at least in its present state, it has not power distinctly to exhibit. 
§ 7* On Analytic Crystals. 
I now come to describe a property of crystals which I met with while employed in 
pursuing the above investigation. This is the power which certain crystals have of 
analysing polarized light in a manner analogous to the tourmaline ; for which reason 
I shall propose for them the name of Analytic Crystals. 
If I am not mistaken, this property has been hitherto confined to the tourmaline 
and a few other natural minerals ; and it has not been known that their effects could 
be imitated, much less surpassed, by crystals artificially made. I trust, therefore, 
that it will be of some interest to describe a method of procuring such crystals. 
In the following experiments it will be understood that the analysing plate of the 
microscope (or the polarizer next the eye) is removed. 
1. A good example of this kind of crystal is obtained by dissolving the sulphate of 
chromium and potash in tartaric acid by the aid of heat. A drop of this solution 
placed on a plate of glass soon yields by evaporation filmy crystals, which very fre- 
quently have the characteristic property of tourmaline : that is to say, that if polarized 
light is transmitted through them, in one position they suffer it to pass freely, but if 
they are turned round 90°, they arrest and absorb it entirely. 
When the experiment has been successfully conducted, the crystals will not in this 
position allow the smallest portion of light to pass. 
If now we consider the extreme thinness of these crystalline films, it will appear 
how energetic must be their action upon light : since although white and transparent, 
they are able to produce an absorption equalling that of the best tourmalines, not- 
withstanding that the effect of the latter is aided by their natural dark colour. 
But if these crystals are analogous to the tourmaline, they must have the power 
which that substance has of analysing the light that has been transmitted through 
other crystals. 
Accordingly, if we place in the path of the polarized ray a plate of sulphate of lime 
of a proper thickness, the crystal, which before absorbed the light and appeared 
black, becomes splendidly coloured with that colour which the sulphate of lime pro- 
duces, and which a tourmaline would show if it were employed as an analysing plate. 
On reversing the polarization of the ray (or turning round the crystal), the com- 
plementary tint appears. The same results occur if the crystal is employed in the 
