tra?ismitted through crystallized Bodies. igg 
was the same, though the thickness of the plate could not 
exceed the 4001 h part of an inch. The colours indeed were 
rendered more brilliant by the increased transparency of the 
agate, but in other respects they experienced no change. In 
another specimen, of which it is unnecessary to give a parti- 
cular description, the coloured image formed an angle of about 
34" with the colourless pencil, and the different veins produced 
the same colour at different angles of incidence. 
In attempting to explain these appearances, I at first ima- 
gined that the colours arose from the polarisation of the trans- 
mitted rays, and that they were analogous to the colours of 
plates of mica and topaz which I have described in another 
place. I found, however, from several experiments, that the 
coloured image is equally distinct in every position of the 
agate; that it is* alike produced by polarised or depolarised 
light, and that it suffers no change either when examined by 
a plate of agate or by a doubly refracting crystal. 
The phenomenon which we have described must therefore 
be considered as a new case of the production of colour, and 
though we do not pretend to point out its cause, yet it obvi- 
ously depends upon a particular structure which is possessed 
only by some portions of the agate, and admits of such varia- 
tions as to produce the same colours at different angles of 
incidence. 
IV. On the depolarisation of Lights 
In the fourth book of my Treatise on new Philosophical 
Instruments, I have already shewn that almost all transparent 
