1816.] _ during the Year 1815. Fp 
reflected from the surface of diaphanous bodies, he examined) the 
metals, and found that polarization was not produced by reflection 
from them, at least in the same manner as from diaphanous bodies. 
But Dr. Brewster afterwards discovered that, when a ray of light 
already polarized is reflected several times from the surface of plates 
of silver or gold, it is modified in such a way that, when analyzed 
by means of a prism of Iceland spar, it divides itself into two diffe- 
rently coloured pencils. Biot, on repeating the experiment, ob- 
served that the colours of the pencils were precisely the same with 
the coloured rings observed by Newton. These observations did not 
coincide with those of Dr. Brewster ; but upon mentioning the sub- 
ject to M. Arago, that Gentleman stated that he had obtained re- 
sults similar to those of Dr. Brewster, and furnished Biot with a 
plate of silver by means of which that philosopher was enabled to 
observe similar results. Surprised at this difference, he investigated. 
the subject with care, and found that the phenomena depended on 
the way in which the metallic plate had been polished. There are 
two ways of polishing metallic plates; by hammering and by fric- 
tion. When the former mode is followed, the phenomena observed 
by Biot are obtained ; when the latter, the phenomena observed by 
Dr. Brewster. Biot at last ascertained that a metallic surface 
polished by friction produces two distinct effects upon light. It 
gives to a part of the incident light what he calls moveable polariza- 
tion, the same which is produced by a thin crystallized plate. This 
occasions the series of coloured rings of Newton. It gives also to 
the white incident light a fixed polarization in the plane of inci- 
dence, the same as is produced by a thick crystalline plate. The 
first of these polarizations is only sensible in particular positions 
when the metallic plate is polished by friction. Hence the reason 
why it was not observed by Dr. Brewster; but it is strong when the 
plate is polished by hammering, and accordingly it was observed by 
Biot. 
3. M. Biot showed long ago that when light traverses certain 
erystals, the repulsive force which produees the extraordinary 
polarization acts with more intensity on the violet molecules than 
on the blue, more on the blue than on the green, and so on, acting 
with least intensity upon the red ray. It is natural to conclude that 
the extraordinary refraction acts in the same manner on the mole- 
cules of light, since it is intimately connected with polarization, 
In a memoir published in the Annales de Chimie for June last 
(vol. xciv. p. 281,) he has shown that this law holds with respect to 
Iceland crystal, and indeed all crystals in general. 
I shall now give a short account of the discoveries on this subject 
published by Dr. Brewster during the year 1815. 
1. He found that the glass tears formed by dropping melted glass 
into water, and commonly called Prince Rupert’s drops, have the 
property of depolarizing light like crystallized bodies. He observed 
cleavages in these glass drops, as in crystals. When sufficiently 
heated, and allowed to cool slowly, they lose the property of depo# 
