X. On the communication of the structure of doubly refracting 
crystals to glass, muriate of soda, fluor spar , and other sub- 
stances , by mechanical compression and dilatation. By David 
Brewster, LL. D. F.R. S. Lond. and Edin. In a letter 
addressed to the Right Hon. Sir Joseph Banks, Bart, G.C.B. 
P.R.S. 
Read February 29, 1816. 
Dear Sir, 
Notwithstanding the numerous discoveries which have 
lately been made relative to the polarisation of light, and the 
optical phenomena of crystallized bodies, not a single step 
has yet been made towards the solution of the great problem 
of double refraction. What is the mechanical condition of 
crystals that form two images and polarise them in different 
planes ; and what are the mechanical changes which must be 
induced on uncrystallized bodies in order to communicate to 
them these remarkable properties, are questions which are as 
difficult to be answered at the present moment, as they were 
in the days of Huyghens and Newton. 
In the frequent attempts which I have made to obtain a 
solution of these difficulties, the polarisation of light by oblique 
refraction was the only phenomenon that seemed to connect 
itself with the inquiry ; but the hopes of success which this 
fact inspired, were soon found to be delusive, and the subject 
resumed its former impregnable aspect. A new train of 
experiments, however, has enabled me not only to give a 
