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III. Experiments on the depolarisation of light as exhibited by 
various mineral , animal, and vegetable bodies, with a reference of 
the phenomena to the general principles of polarisation. By 
David Brewster, LL. D. F. R. S. Edin . and F. S. A. Edin. 
In a letter addressed to the Right Hon. Sir Joseph Banks, Bart . 
K. B. P. R. S. 
Read December 15, 1814. 
Dear Sir, 
Towards the end of the year 1812, when I was engaged in 
examining the light transmitted through diaphanous bodies, 
I discovered the property which many of them possessed of 
depolarising the rays of light, or of depriving them of the po- 
larity which they had received, either by reflection from the 
surface of a transparent body, or by transmission through a 
plate of agate. A short account of these experiments, which 
were exhibited to many of my friends in Edinburgh, was 
soon afterwards published in my treatise on new philosophical 
instruments. 
As this singular property was possessed by numerous sub- 
stances that exhibited no marks of double refraction, and even 
by animal and vegetable products, such as horn, tortoise- 
shell, and gum Arabic, it appeared necessary to distinguish 
it by a new name, and to refer it to a species of crystalliza- 
tion different from that of doubly refracting cr} r stals. The 
circumstance, however, of agate and Iceland spar possessing 
