of doubly refracl'wg Crystals . 127 



part of the crystal, having a different structure from that which 

 transmits the light of the other image. In the carbonate of 

 barytes, however, where the transparency of the crystal is imper- 

 fect, one of the images is nebulous and imperfect; ami as the 

 same phaenomenon is exhibited in the agate and in the im- 

 perfectly transparent crystals of the nitrate and carbonate 

 of potash, we are entitled to conclude, that the light whicli 

 forms the imperfect image is transmitted through the imperfect 

 structure ; while the light which forms the bright image is 

 transmitted through a structuie of a more perfect kind. The 

 imperfect transparency, therefore, of the crystal, and the nebu- 

 lous character of one of the images, can be considered in no 

 other relation than that of cause and effect. 



From the optical properties of the agate, this conclusion de- 

 rives a still higher degree of probability. The two images formed 

 by this mineral are not similar to each other, like those of cal- 

 careous spar, though they possess exactly the same properties. 

 One of them is bright and distinct, and the other is a mass of 

 nebulous light. Now it happens that the agate possesses tv/o 

 different kinds of structure, corresponding to the characters of 

 its two images, and distinctly perceptible even to the naked eye. 

 One of these structures is composed of small serpentine lines 

 like the figures 3333, resembling the surface of water ruffled by 

 a gentle breeze ; and I have a specimen in my possession, one- 

 half of which has much larger serpentine lines than the other. 

 The light which passes through the serpentine lines, is that which 

 forms the nebulous image ; while that which passes between 

 them forms the distinct image. This ma} be demonstrated by 

 a variety of experiments. 



When the light is transmitted through a part of the agate tiiat 

 has the largest serpentine lines, the nebulous image has an ap- 

 pearance different from that which it has when the light is 

 transjnitted through the other part where the serpentine lines- 

 are smaller. If the agate is inclined in the direction of the ser- 

 pentine lines, so as not to prevent the rays from passing between 

 them, the bright image will be distinctly visible as before ; but 

 when the ;igate is inclined in a direction at right angles to this, 

 so as to prevent the rays from passing between the serpentine 

 lines, the whole of the transmitted light is nebulous. Hence it 

 follows, that the nebulous image is produced by the imperfect 

 structure of the agate, indicated by the serpentine lines ; while 

 the bright image is produced l)y a structure the same as that of 

 all other transparent bodies. 



The curvature of the nebulous light, in some specimens of 

 -agate with incurvated vcins^ and its constant parallelism to the 



laminae. 



