192 Dr. Brewster on the Affections of Light 
the sections of a succession of laminse that are sometimes 
parallel, but in general concentric. These laminae are often of 
a milky white colour when seen by reflected light,' and some- 
times nearly as transparent and colourless as glass, and the 
white laminae commonly alternate with the transparent ones. 
The laminae which are white when seen by reflected light, are 
brown by transmitted light, and the intensity of this brown 
colour increases with the thickness of the plate of agate. The 
transparent laminae exhibit three varieties of structure. 
The first variety, which appears to be the coarsest, consists 
of a number of small serpentine lines like the figures 333333, 
lying parallel to each other, and closely resembling the sur- 
face of standing water when ruffled by a gentle breeze, or 
the sandy bottom of a slow moving stream. These serpentine 
lines are always arranged in a direction parallel to the laminae, 
and are seen very distinctly even when the agate is so thin as 
the 150th part of an inch. 
The second variety of structure differs from the first, only 
in the serpentine lines having a much smaller size; and the 
laminae which have this structure appear the finest and most 
transparent. 
The third variety has no serpentine lines, and does not ap- 
pear to differ from other semi-transparent bodies. It admits 
the light more copiously in all directions than any of the other 
structures, and as it does not polarise it in a similar manner, 
we may consider it as possessing, in a different way, that 
kind of crystallisation which polarises the incident light by 
separating it into two pencils. 
The white veins sometimes exhibit the first variety of struc- 
ture, but in several specimens the veins appear to be fibrous 
