64 Dr. Brewster on the effects of simple pressure , &c. 
polarised ray, I saw that it did not in the least degree depo- 
larise it. I then exposed the included jelly to a considerable 
pressure, and it instantly restored the evanescent image, and 
exhibited, in a faint degree, the complementary colours. This 
plate was not more than ^th of an inch thick. 
From these experiments and others, which have been re- 
peated under various modifications, it follows : 
1st. That soft animal substances which have no particular ac- 
tion upon light acquire, from simple pressure , that peculiar structure 
which enables them to form two images polarised in an opposite 
manner , like those produced by all doubly refracting crystals , and 
to exhibit the complementary colours produced by regularly crystal- 
lized minerals. 
2d That soft animal substances , which already possess the pro- 
perty of depolarising light , receive from simple pressure such a 
modification in their structure as to enable them to exhibit , in a 
very brilliant manner , the complementary colours produced by 
crystallized minerals. 
3d. That soft animal substances which only depolarise a portion 
of the incident ray , have their depolarising structure completed by 
simple pressure. 
The extension of these experiments to other soft substances, 
to hard bodies when in a fluid state, and to fluids themselves, 
may probably lead to still more interesting results. 
I have the honour to be, Dear Sir, 
your most obedient humble servant, 
DAVID BREWSTER. 
Edinburgh, January 3, 1815. 
To the Right Hon. Sir Joseph Banks, Bart. K. B. P. R, S. 
