OPTICAL PHENOMENA OF CRYSTALS. 225 



cult 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. 



SECT. 7 On Analytic Crystals. [ now come to de- 

 scribe a property of crystals which I met with while em- 

 ployed in pursuing the above investigation. This is, the 

 power which certain crystals have of analyzing 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 arti- 

 ficially 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 analyzing plate of the microscope (or the pola- 

 rizer 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 tar- 

 taric 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 frequently have the charac- 

 teristic property of tourmaline : that is to say, that if 



Q 



