OF CARBONATE OF 13ARYTES. 291 



separated, and are extremely Imperfect. When the angle of 

 incidence at the first surface of the prism is increased, the 

 images become more and more distinct, and better separated ; 

 but, by diminishing the angle of incidence, all the images ap- 

 proach one another, and are confounded into one mass of ne- 

 bulous light. 



With a plate of Carbonate of Barytes, which was about two- 

 tenths of an inch thick, and wliich had its surfaces at right 

 angles to the direction of the longitudinal joints, the image of 

 a candle was a large circular mass of light, when the incidence 

 was perpendicular. By inclining the plate, this mass was chan- 

 ged into an annular image : By increasing the inclination, it as- 

 sumed the form of a crescent, and at a considerable angle of 

 incidence, it was separated into three imperfect images, or cir- 

 cular arches of nebulous light, similar to those which were 

 seen with the fifth prism. The middle image, which was the 

 brightest, consisted of the ordinary and extraordinary image, 

 which were not separated, in consequence of the parallelism of 

 the refracting surfaces. In one position of the plate, these 

 arclies were crossed by other three similar, arches, inclined to 

 the first at an angle of 10° or 12°. 



The phenomena which have now been described, differ in 

 several respects from those which are presented by the agate. 

 In the Carbonate of BaryteSy the two images are distinctly se- 

 parated, and are, therefore, formed by two separate refractive 

 powers ; whereas in the Agate, the bright image is placed in 

 the centre of the nebulous mass.. In the Carbonate of Barytes^ 

 the imperfect image occupies a small space ; but in the Jgate, 

 it is an elongated mass of light, extending about 7^° in length, 

 and about 1 ° 7' in breadth, on each side of the bright image. 

 These differences, however, are probably owing to the different 

 ways in which the two minerals have been cut; but it is not 



easy 



