298 OV THE STRUCTURE OP 



Wlien I discovered the property possessed by the agate, of 

 forming a bright and a nebulous image, and of polarising them 

 in an opposite manner, like all doubly refracting crystals, I 

 was sufficiently aware of the conclusions which it authorised * ; 

 but as no other crystallised body exhibited analogous pheno- 

 mena, I contented myself with stating these conclusions as 

 mere conjectures, which required the sanction of numerous ex- 

 periments. 



In the carbonate of barytes, however, and in the nitrate and 

 carbonate of potash, we ai'e presented with properties analo- 

 gous to those of the agate, and are therefore enabled to resume 

 this subject, with that confidence which can only be derived 

 from multiplied observations. 



When we examine the two images formed by calcareous 

 spar and other perfectly transparent crystals, we find that they 

 have the same magnitude, and are equally luminous and di- 

 stinct. There is, therefore, no circumstance which can lead us 

 to suppose, that the light which forms the one image passes 

 through a part of the crystal, having a different structure from 

 that which transmits the light of the other image. In the car- 

 bonate of barytes, however, where the ti'ansparency of the cry- 

 stal is imperfect, one of the images is nebulous and imperfect ; 

 and as the same phenomenon is exhibited in the agate and in 

 the imperfectly transparent crystals of the nitrate and carbonate 

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

 forms the imperfect image is transmitted through the imper- 

 fect structure ; while the light which forms the bright image, 

 is transmitted through a structure of a more perfect kind. 

 The imperfect transparency, therefore, of the crystal, and the 



nebulous 



• See Phil. Trans. Lond. 1813, Part 1. p. 101. 



