1^6 Dr. Brewster on the Affections of Light 
the agate enable us to give a satisfactory explanation of so m 
singular appearances exhibited by that mineral. 
In the specimen shewn in Plate V., fig. 5, the black lines 
represent the veins, and consequently the direction of the 
lamina, and the dotted lines ah, ac, cm, ck, &c. are drawn 
through the vertices of the angles made by the veins ; and 
consequently by the laminae whenever they change their direc- 
tion. When light is transmitted through a piece of agate of 
this description, the planes Aacm, mck, ngj, nhg have the appear- 
ance of being differently inclined to one another, and transmit 
different quantities of light. If the veins and the laminae pre- 
served the same inclination to the surface of the plate of agate 
v,^hen they changed their direction at the lines ac, cm, de, the 
piienomenon which has been mentioned could not take place; 
but whenever the laminas change their diiection, their inclina- 
tion to the surface likewise changes, and therefore the inten- 
sity of the transmitted light experiences a corresponding 
variation as the rays have to traverse different lengths of the 
imperfectly transparent lamincC. 
When the veins and the laminje are incurvated like those 
in the portion Aacm, their inclination likewise changes, but as 
this change is gradual and not sudden, as in the former case, 
the intensity of the transmitted light suffers a gradual variation, 
and the portion Aacm has the appearance of being concave. 
When the laminae therefore are arranged in a circular form, 
they will resemble a number of dimples, the apparent con- 
cavity of which will in some cases depend on the curvature 
of the laminae, and will exhibit the phenomenon of the liarn^ 
mered agate. 
