6 REPORT—1840. 
thus formed are of extreme beauty, and afford by transmitted light co- 
lours of infinite beauty and variety, surpassing anything produced in 
works of art. They have the effect of directing, as it were, the com- 
pound surface of the solar spectrum, or of sifting and separating the 
superimposed colours, in a manner analogous to what is produced by 
colours and absorbing media. Sir D. Brewster has succeeded indeed 
in producing one or more bands of white light incapable of decompo- 
sition by the prism, and there can be no doubt that they will be found 
to exercise a similar or an analogous action on the heating rings of the 
thermometric spectrum. In the decomposed glass from St. Andrew's 
a change of a very different kind is effected. In some cases the sili- 
ceous and the metallic elements of the glass are separated in a very sin- 
gular manner, the particles of silex having released themselves from the 
state of constraint produced by fusion and subsequent cooling, and ar- 
ranged themselves circularly round the centre of decomposition ; while 
the metallic particles, which are opake, had done the same thing in cir- 
cles alternating with the circles of the siliceous particles. This restora- 
tion of the silex to its crystalline state is proved by its giving the co- 
lour of polarized light, and possessing an axis of double refraction. 
The preceding notice was illustrated by diagrams and specimens of 
the different kinds of glass referred to. 
On the Rings of Polarized Light produced in specimens of Decomposed 
Glass. By Sir Davip BRewsTER. 
In the course of a series of experiments on the connexion between 
the absorption of light and the colours of thin plates, published in the 
Phil. Trans., 1837, the author accidentally observed under the polar- 
izing microscope certain phenomena of polarized tints, of great beauty 
and singularity. These tints were sometimes linear and sometimes cir- 
cular, and in some specimens they formed beautiful circular rings, tra- 
versed by a black cross, resembling the phenomena of mineral crystals, 
or those produced by rapidly cooled circular plates or cylinders of glass. 
Having found in the decomposed glass from St. Andrew's that the sili- 
ceous particles had resumed their position as regular crystals, and ar- 
ranged themselves circularly round the centre of decomposition, he was 
led to suppose that this was the cause of the phenomena, and that 
the rings were the effect of the double refraction of the minute cry- 
stals. A few experiments, however, overturned this hypothesis, and he 
was soon satisfied, by a little further investigation, that the phenomena 
arose wholly from the polarization of the transmitted light by refrac- 
tion, the splendid colours being entirely those of thin plates, which 
were sometimes arranged so as to have the appearance of concentric 
rings. The structure by which these effects were produced was com- 
pared by the author to a heap of very deep watch-glasses laid one above 
another. When the thin films were arranged longitudinally, and were 
inclined to the general surface of the plate, so as to transmit the rays ob- 
liquely, the light was still polarized, but only in one plane, namely, a 
