1/8 Dr. Brewster on the co m mimical io n of double refraction , &c* 
ring the polarisation of the two pencils to the operation of 
some peculiar fluid. The new property of radiant heat which 
enables it to communicate double refraction to a distant part 
of a plate of glass, where the heat does not reside in a sensible 
state ; — the existence of a moveable polarity in glass, whe- 
ther the doubly refracting structure is communicated trans- 
iently or permanently; — and the appearance of regular cleav- 
ages varying with the direction of the axes of double refrac- 
tion, are facts which render it more than probable that a 
peculiar fluid is the principal agent in producing all the phe- 
nomena of crystallization and double refraction. 
There is one fact, however, which forms a fine connection 
between the aberration of the extraordinary ray and the prin- 
ciples established in this Paper. It has been demonstrated b}^ 
an eminent English philosopher,* that every undulation must 
assume a spheroidal form when propagated through a minutely 
stratified substance, in which the density is greater in one 
direction than another, and I have proved by experiment 
that such a substance actually possesses the property of double 
refraction. This singular coincidence will no doubt be re- 
garded as an argument in favour of the undulatory system. 
I have the honour to be, &c. 
DAVID BREWSTER. 
To the Tight Hon. Sir Joseph Banks , Bart. 
G. C. B. P. T. S. 8fc. fyc. Sfc. 
* See Quarterly Review, Vol II. 
From the Press of 
W. BULMER & Co. 
Cleveland-row, St. James’s, 
London. 
