of Light mthin Solid and Fluid Bodies. 105 



incapable of further undergoing cpipolic dispersion;" but as 

 the dispersing action is not thus Hmited, that conclusion must 

 be incorrect. Sir John Herschel, indeed, has deduced this 

 result from direct experiment with a plate of glass immersed 

 vertically in a quiniferous solution. In this case he could 

 perceive no trace of colour, either at the ingress or egress of 

 the epipolized beam which was incident upon the plate. Sir 

 John does not mention the distance of the plate from the epi- 

 polizing stratum. If the distance was small, we are confident, 

 from direct experiment, that the blue tint would have been 

 seen ; but if the distance was considerable, then the beam, 

 incident upon the glass, must have been previously shorn of 

 all its dispersible rays. 



In examining the blue rays themselves, Sir John found that 

 they consisted of a " small per-centage of rays, extending over 

 a great range of refrangibility." They formed, however, a 

 continuous spectrum deprived of the less refrangible red, 

 nearly of the whole orange, and all the yellow ; a rich and 

 broad band of fine green light, slightly fringed with red, passed 

 into a copious indigo and violet without the intermediate blue. 



The comparatively feeble light of the dispersed blue rays 

 renders it difficult to ascertain their susceptibility of being a 

 second time dispersed. Sir John Herschel could not obtain 

 any indication of this susceptibility ; but we have no doubt 

 that, with condensed light, their second dispersion will be 

 discovered : and we are led to this opinion by the fact^ that 

 Sir John believed that the epipolic dispersion takes place in 

 all directions, and therefore expected to discover a second 

 dispersion under circumstances in which, according to my 

 experiments, it could not be found. 



Sir John has clearly shown, that the light is dispersed out- 

 wards as well as laterally ; but as he was conversant only with 

 the phaenomena of a narrow blue line, and had not seen the 

 blue cone of rays dispersed from the cone of condensed light, 

 he could not be aware of the changes which take place in its 

 colour while the eye passes from the azimuth of 90° to that 

 of lOO''. 



These changes are very decided, and will be understood 

 from fig. 2, in which MNOP is a horizontal section of the 

 vessel containing the solution; RR' a beam of solar light in- 

 cident upon an achromatic lens LL, and condensed into the 

 luujinous cone ACB. Now, the blue colour produced by the 

 first stratum, next to the side AB, is exceedingly strong, and 

 that which occupies the rest of the cone ACB comparatively 

 faint. When we view the bright blue stratum in the direction 

 NM, or in the azimuth of 90°, the tint is very brilliant, because 



