transmitted through crystallized Bodies. 263 



.hibit the seco7id variety of structure, though, the small serpentine 

 lines are not so distinctly marked as in other specimens. The 

 two veins AB, CD, are both white when seen by reflected light. 

 The breadth of AB is one-tenth of an inch, and its radius of 

 curvature U inch, and it consists oi four smaller veins mn, op, 

 qr, ivx. The light reilected by mn is a paler white, and the 

 light transmitted by it a lighter brown than in the other parts 

 of the vein. The light which op reflects is of a brighter white,. 

 and that which it transmits of a deeper brown than in the other 

 parts of the vein, and at the junction of mn and op there are 

 several tufts of fdires of the same character as op. The other 

 divii.ions of the vein qr and wx are of an intermediate character 

 between mn and op. The vein CD resembles the division nni^ 

 and possesses, like AB, the fibrous structure already described. 

 The thickness of the plate AEDC is one-fiftieth part of an inch ; 

 AC is three-tenths of an inch ; and a line AC forms an angle 

 of about 25" with a plane perpendicular to the larninre. 



Figine 2, represents anoth; r specimen of agate of a difterent 

 character. It consists of transparent portions AB, BC, CD, 

 DE, EF, separated from each other by white veins B/', Cc, &;c. 

 and distinctly exhibiting; the second variety of structure ; and 

 of other transparent ijortious FG, GH, HI, IK, KL, separated 

 by similar veins Gg, Wh, &c. but exhibiting the fiist variety of 

 structure. All the veins possess a structure approaching to that 

 of the first variety, G^, K^ exceeding the rest in the intensity of 

 the light which thev reflect and transmit. 



If we measure the quantity of light transmitted through a 

 plate of agate containing veins, it vvili be found to be a maximum 

 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 transparency, and 

 perfectly free from veins, the same result will be obtained, the 

 intensity of the light being a maximum when its direction is pa- 

 rallel to tliat of the lamin?^. 



If AB, fig. 3, be a section of the specimen of agate repre- 

 sented in fig. 1, and mn, op, the direction of the laminas inclined 

 25"^ to tlie 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 di- 

 minution : 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 little diminution, and its colour is not sensibly 

 altered. 



These 



