DOUBLY REFRACTING CRYSTALS. 299 



nebulous 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 calcareous spar, though they possess exactly the same pro- 

 perties. One of them is bright and distinct, and the other is 

 a mass of nebulous light. Now it happens, that the agate pos- 

 sesses two 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 sur- 

 face of water ruffled by a gentle breeze ; and I have a speci- 

 men in my possession, one-half of which has much larger ser- 

 pentine 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 may be demonstrated by a variety of experi- 

 ments. 



When the light is transmitted through a part of the agate 

 that has the largest serpentine lines, the nebulous image has 

 an appearance different from that which it has when the licht 

 is transmitted through the other part where the serpentine 

 lines are smaller. If the agate is inclined in the direction of 

 the serpentine lines, so as not to prevent the rays from passing 

 between them, the bright image will be distinctly visible as be- 

 fore ; but when the agate is inclined in a direction at ri^ht 

 angles to this, so as to prevent the rays from passing between 

 the serpentine lines, the whole of the transmitted light is ne- 

 bulous. Hence it follows, that the nebulous image is produ- 

 ced by the imperfect structure of the agate, indicated by the 



serpentine 



