ic)4j -Dr. Brewster on the Affections oj Light 
of other transparent portions FG, GH, HI, IK, KL, separated 
by similar veins Gg^ H/i, &c. but exhibitinjg^ first variety of 
structure. All the veins possess a structure approaching to 
that of the first variety, Gg, exceeding the rest in the in- 
tensity of the light which they reflect and transmit. 
If we measure the quantity of light transmitted through a 
plate of agate containing veins, it will be found to he a maxi- 
mum when the direction of the incident rays is parallel to the 
interior surfaces of the veins. When the light, however, is 
transmitted through a part of the agate of an uniform trans- 
parency, and perfectly free from veins, the same result will 
be obtained, the intensity of the light being a maximum when 
its direction is parallel to that of the lamincC. 
If AB, fig. 3, be a section of the specimen of agate repre- 
sented in fig. 1, and mn^ op, the direction of the laminae inclined 
25° to the surfaces of the plate, rays of light incident in the 
direction RS parallel to mn are more copiously transmitted 
than when they are incident in any other direction. When the 
pencil of light falls in the direction TV, its lustre suffers a 
great diminution : the light gradually assumes a red colour, 
and vanishes altogether when the obliquity is considerable. 
But if the pencil is incident at the same angle on the opposite 
side, as PQ, its lustre suffers very lille diminution, and its 
colour is not sensibly altered. 
These facts admit, to a certain extent, of an easy explana- 
tion if we suppose that the plate of agate consists of lamina 
mn, op imperfectly transparent, alternating with laminae cd, ef, 
which are more pervious to light, a structure which is indicated 
by the existence of a bright and a nebulous image. In this 
case the intensity of the light will obviously be a maximum 
