XVIII. On the Laws which regulate the Distribution of the 
Polarising Force in Plates, Tubes, and Cylinders of 
Glass, that have received the Polarising Structure. 
By Davin Brewster, LL. D. F. B.S. Lonp, & Eni. 
( Read June 17. 1816.) 
I. the Philosophical Transactions for 1816, I have described 
at great length the various. phenomena which are exhibited by 
glass and other substances to which the property of double re- 
fraction has been communicated by heat, by rapid cooling, by 
evaporation, or by mechanical compression and dilatation. In 
pursuing the same subject, I have observed many singular facts 
respecting the developement of new axes, by a:change in the 
form and condition of the plates; and by submitting the phe- 
nomena to accurate measurement,. I have succeeded in deter- 
mining the laws which regulate the distribution of the polari- 
‘sing force. A brief account of these results will form the sub- 
ject of the following paper.. 
1. On Plates of Glass with One Axis of Polarisation: 
If we take a plate of glass perfectly circular, and communi- 
cate to it the polarising structure, either transiently, by the 
transmission of heat from its circumference to its centre, or 
permanently, by cooling it rapidly, when it has been made red 
hot, we shall find that it will exhibit, when exposed to pola- 
rised light, a system of rings traversed by a black rectangular 
cross. This system of rings is precisely the same, both in ap- 
pearance and in the character of its tints, as the system seen 
: yA a along 
