C6o 3 
V. On the effects of simple pressure in producing that species of 
crystallization which forms two oppositely polarised images , and 
exhibits the complementary Colours by polarised light. 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 January 19, 1815. 
Dear Sir, 
In prosecuting the experiments on the depolarisation of light, 
which you lately did me the honour to lay before the Royal 
Society, I have been led to the discovery of a remarkable pro- 
perty of soft transparent solids, in virtue of which they ex- 
hibit, by simple pressure, all the optical qualities of doubly 
polarising crystals. In the paper on depolarisation to which 
I have now alluded, it has been shown that a mixture of bees* 
wax and rosin, when melted and cooled between two plates 
of glass, depolarises a ray which falls upon it at a vertical inci- 
dence, while the same substance, pressed between two plates 
of glass, without the aid of heat, produces no effect when the 
polarised ray falls perpendicularly upon it, but depolarises it 
at an oblique incidence. In this experiment the crystallization 
was not produced by pressure, as the unmelted bees* wax 
was already crystallized; but it is obvious, either that the 
pressure had modified the natural crystallization of the bees* 
wax, so as to enable it to depolarise only at an oblique inci- 
dence, or that its liquefaction between two plates of glass had 
