of Light mtliin Solid and Fluid Bodies. 4<03 



thei/ellow, I'cd, and bright blue varieties wliich I have examined. 

 It occurs chiefly in ihe green fluor from Alston Moor, and in 

 several pink and bluish-yello-w varieties from Derbyshire. In 

 order to observe the phaenomena of dispersion most distinctly, 

 I transmit a condensed beam of the sun's light through the 

 specimen when partially covered with black wax or black 

 velvet. In some specimens, the intromitted beam is partially 

 dispersed in a fine blue tint from every part of the solid which 

 it traverses ; but in other specimens, which are composed of 

 strata of different colours, parallel to the faces of the cube, a 

 very different and a very instructive phenomenon is displayed. 

 The intromitted beam ABC, fig. J, Plate V., is crossed with 

 bands of dispersed light of different colours and of different 

 intensities. In one case, a pink light was dispersed from the 

 stratum close to the surface of incidence ; from the next stra- 

 tum there was no dispersion at all ; this was followed by a 

 narrow stratum, which dispersed a bright whitish light ; then 

 succeeded a stratum of non-dispersing fluor, and alternately 

 dispersing and non-dispersing strata, scattering the fine blue 

 light which has already been mentioned. 



These results, which I have shown to different persons, are 

 incompatible with those obtained by Sir John Herschel with 

 the very same variety of Jluor-spar, He regards the blue 

 dispersed light as strictly an epipolic or superficial tint, — so 

 superficial, indeed, " that it might be referred to a peculiar 

 texture of the surface, the result of crystallization, were it not 

 that it appears equally on a surface artificially cut and po- 

 lished*." Were I to hazard a conjecture respecting the cause 

 of this diff*erence in our results, I would ascribe it to the dif- 

 ferent degrees of light in which the observations were made. 

 While I used a condensed beam of the sun's light. Sir John 

 Herschel seems to have employed chiefly the ordinary light of 

 day. In studying the phaenomena in the solution of quinine, 

 he "exposed it to strong daylight or sunshine;" and in an- 

 other experiment, which pre-eminently required a powerful 

 illumination, he "directed a sunbeam downwards on the sur- 

 face, by total reflexion from the base of a prism," which was 

 in reality inferior to the ordinary sun's light. In the case of 

 fluor-spar, however, he states that the epipolic colour is seen 

 in perfection when "exposed to daylight at a window." In 

 such a feeble light I could not have seen the phaenomena I 

 have described; and it is owing chiefly to the intensity of the 

 light which I employed, that I have been enabled to place it 

 beyond a doubt tliat the blue light dispersed by fluor-spar is 

 reflected from every part of the interior of the crystal, and is 

 • Pliilosophical Transactions, 1845, p. 143. 

 2 D 2 



