300 -ON THE STRUCTURE OP 



serpentine lines ; while the bright image is produced by a 

 structure the same as that of all other transparent bodies. 



The curvature of the nebulous light, in some specimens of 

 agate, with incurvated veins, and its constant parallelism to 

 the laminae, and to the direction of the serpentine lines, give 

 additional probability to this conclusion. 



Here, then, we have a case of the most unequivocal kind, 

 in which one image of a doubly refracting crystal is produced 

 .by one structure, or by one part of the crystal, while the other 

 image is produced by another structure, or another part of the 

 crystal ; and hence we are led to conclude, in general, that the 

 two images exhibited by all doubly refracting bodies, are 

 formed by two different structures, related to some axis or fix- 

 ed line in the primitive crystal. Whether this difference of 

 structure is produced by a difference in the arrangement of 

 the elementary molecules, or is owing to a combination of dif- 

 ferent ingredients, is a point which still remains to be deter- 

 mined. 



The phenomena presented by the agate and the carbonate 

 of barytes, convey still farther information respecting the 

 structure of these imperfect crystals. In one direction, the 

 light transmitted by the agate is wholly nebulous ; the perfect 

 image being converted into a shapeless cloudy mass, and con- 

 founded with the nebulous image. In another direction, one 

 of the images is distinct and perfectly formed ; and, in one 

 specimen, which has the faculty of depolarisation, there must 

 necessarily be two perfect images. In a prism of the carbo- 

 nate of barytes, both the images were imperfect. In a 

 second prism, the one image was nebulous, and the other 

 distinctly formed ; while, in other prisms, there was a ra- 

 pid approximation to two perfect images. Hence it follows, 



that 





