290 ON THE OPTICAL PROPERTIES 



longitudinal joints were perpendicular to the plane of reflec- 

 tion, the light image became extremely distinct, in conse- 

 quence of the nebulous light having now refused to penetrate 

 the prism. 



In a second prism, the nebulous, or most refracted image 

 was more luminous than in the first, and approached to a defi- 

 nite form ; the general shape of the candle being distinctly vi- 

 sible. 



In a third prism, the nebulous image was more luminous 

 than in the last case, and the form of the candle still more di- 

 stinctly seen ; but it had now the appearance of an assemblage 

 of incoincident images. 



In a fourth prism, in which the plane of refraction was pa- 

 rallel to the longitudinal joints, both the images were imper- 

 fect, and the most refracted image was extremely faint. By 

 inclining the prism, an image appeared on each side of the 

 least refracted image ; but they were polarised in the same 

 manner, and were probably analogous to the two images which 

 are frequently seen in specimens of calcareous spar. 



In a fifth prism, which was formed by planes nearly per- 

 pendicular to the longitudinal joints, four images were plainly 

 visible, all of which were imperfect, and consisted of circular 

 arches of nebulous light. The two middle ones, which were 

 the principal images, were equally luminous, and were polari- 

 sed in an opposite manner, like all other double images ; but 

 each of the two outer images was polarised in the same man- 

 ner as the bright image farthest from it. The rnost refracted of 

 the two principal images, was in this case the most perfect of 

 the two, and exhibited a degree of prismatic colour so much 

 greater than the other, that it obviously belonged to a higher 

 dispersive power. When the light enters the prism, and emer- 

 ges from it at equal angles, the four images are not distinctly 



separated, 



