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XIII. On the laws of polarisation and double refraction in regu- 
larly crystallized bodies. By David Brewster, LL.D. F. R. S. 
Lond. and Edin. In a letter to the Right Hon. Sir Joseph 
Banks, Bart. G.C. B. P. R. S . 
Read January 15th, 1818. 
Venlaw , June 1, 1817, 
DEAR SIR, 
In the different memoirs which you have done me the 
honour of submitting to the Royal Society, I have considered 
principally those branches of the polarisation of light which 
relate to the superficial action, or the superinduced properties 
of uncrystallized bodies. In the course of these enquiries, 
my attention was frequently directed to the phenomena of 
regular crystals ; but from the difficulty of procuring proper 
specimens, and the extreme perplexity of the subject, it was 
not till lately that I succeeded in reducing under a general 
principle all the complex appearances which result from the 
combined action of more than one axis of double refraction. 
Before I proceed to trace the steps which have conducted 
me to this general law, I must entreat the indulgence of the 
Society, while I attempt to give a brief and rapid view of the 
present state of our knowledge respecting the laws of double 
refraction. They will thus be able to appreciate more cor- 
rectly the relative value of those successive generalisations 
by which this subject has been raised to one of the most 
interesting departments of physical science. 
