2i8 Dr. Brewster on the Affections of Light, &c. 
segments are distinctly visible, excepting in caoutchouc, where 
the colours are extremely faint. 
It is highly probable that the coloured rings will be found 
in a still greater number of crystallized bodies. I have sought 
for them in vain in the diamond, native orpiment, Iceland spar, 
fluor spar, muriate of soda, carbonate of lead, carbonate of 
barytes, the sclerotic coat of the eye, the crystalline lens, and 
a great variety of other bodies, and in some of these with 
so much care, that they could scarcely have escaped my 
notice, if they did exist. It therefore still remains to be de- 
termined, what kind of crystallization is necessary to their 
production, and what relation exists between the magnitude 
of the rings and the refractive power of the body which pro- 
duces them. In some of the experiments already described, 
the diameters of the rings seem to vary as (m — i but the 
anomalies exhibited by amber and nitrate of potash completely 
prove that this is not the law by which their magnitude is 
regulated. 
I have the honour to be. 
Dear Sir, 
your most obedient humble servant, 
DAVID BREWSTER, 
To Sir Humphry Davy, LL. D., &c. &c. 
