256 On some Properties of Light. 
told that in India houses, nothing but white plastered walls © 
is to be seen. But a tally made of jooaga requires to be 
changed every three days: whereas kus will remain good 
the whole hot season ; which generally lasts four months.” 
XLIX. On some Properties of Light. By Davin Brews 
ster, LL.D. F.R.S, Edin. Ina Letter to Sir H. Davy, 
LL.D. F.R.S.* 
DEAR On © ee been for some time engaged ina 
series of experiments on the phznomena of light arising 
from its transmission through diaphanous bodies, I have 
taken the liberty of communicating to you, for the infor 
mation ofthe Royal Society, a short and general account 
of the results of my inquiries. In the narrow compass of 
a letter, it would be impracticable to include the various 
details of these experiments ; the particular methods of ob- 
servation that were employed; or the numerical results 
which I have obtained for the refractive and dispersive 
powers of nearly two hundred substances. As these will 
form part of a separate work, in which I am now engaged, 
I shal] confine myself at present to some of those results 
which appear to be most interesting, either from their no- 
velty or importance. 
1, On a new Property of refracted Light. 
As you are already well acquainted with the optical pro- 
perties of doubly retracting media, and the analogous pro- 
perty of reflected light discovered by Malus, it will be un- 
necessary to take any notice of these phenomena. After 
repeating the experiments of Malus, and measuring severak 
of the angles of incidence at which this property was com- 
municated to light by reflection from different substances, 
I made a variety of experiments, with the view of discover- 
ing 1f a similar character could be impressed upon hght by 
its transmission through bodies, either wholly or imper- 
fectly transparent. All these experiments afforded no new 
result, and every hope of discovering such a property was 
extinguished, when my attention was directed to a singular 
appearance of colour in a thin plate of agate. This plate, 
bounded by parallel faces, is about the fifteenth of an inch 
thick, and is cut ina plane perpendicular to the lamin of 
which it is composed. The agate is very transparent, and 
* From the Philosophical Transactions for 1819, part i. >a 
gives 
