198 Dr, Brewster on the Affections of Light 
tion of the agate the colours of these portions instantly change, 
a particular colour being always produced in the same por- 
tions at a particular angle of incidence. The veins Gg, 
Iz, and Ki are, however, green when the surrounding portions 
are red, and red when the surrounding portions are green, 
from which it follows that these veins produce a particular 
colour at a different angle of incidence from the adjacent por- 
tions. In another specimen of agate, very like the preceding, 
the same phenomena are distinctly visible, and the coloured 
image forms the same angle with the common image. In a 
third specimen, belonging to Robert Ferguson, Esq. of Raith, 
the colours are exhibited in the most splendid manner. A 
semitransparent and irregularly elliptical zone, about six and a 
half inches in circumference and three tenths of an inch broad, 
has the first variety of structure, and forms the coloured image 
at a distance of from the common image. 
In the specimen represented in Plate V., fig 1, the colours 
are visible only in the vein AB ; but here the angle of the 
first coloured image with the common image is 28®, while 
that of the second image, which is very faint, is only a little 
greater. The other vein CD, which to all appearance has 
the same structure as AB, and which differs from it only 
in being a little thinner, exhibits no colours ; but there is a 
small stripe st at its edge where the colours are very dis- 
tinct. This circumstance induced me to think that the colours 
depended on the thickness of the plate, as well as upon 
its structure ; but upon grinding a hollow place mvw in the 
vein AB, so as to make the agate remarkably thin, I found 
that it gave the same colours as before. A similar experi- 
ment was made with another piece of agate, and the result 
