114 SIR DAVID BREWSTER ON THE DECOMPOSITION AND DISPERSION OF LIGHT 



an inch, of a vivid and nearly uniform blue colour over its whole breadth;"* but 

 upon " directing a sunbeam downwards on the surface, by total I'eflection from 

 the base of a prism, a feeble blue gleam was observed to extend downwards below 

 this vivid line to nearly half an inch from the surface, thus leaving it doubtful 

 whether some small amount of dispersion may not be effected in the interior of 

 the medium at appreciable depths." By using condensed solar light, this doubt 

 is immediately removed, and the phenomenon ranks itself as one of internal dis- 

 persion, differing only in the law of its intensity from those which I have already 

 described. In the one the dispersible rays are thrown gradually, in the other 

 quickly, from the intromitted beam, — a phenomenon to a great extent identical 

 with what takes place in the analogous phenomena of absorption. 



If the dispersing action of the solution were rigorously confined to a stratum 

 the fiftieth of an inch thick, it would have followed, of necessity, that " an epi- 

 polized beam of light (meaning thereby, a beam which has been once transmitted 

 through a quiniferous solution, and undergone its dispersing^ action) is incapable of 

 further undergoing epipoUc disjjersionr but as the dispersing action is not thus limited, 

 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 epipolising stratum. 

 If the distance was small, we are confident, from du'ect 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, Su* 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, 

 neai'ly 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 intennediate 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 Her- 

 schel 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 cfrcumstances in which, according to my experiments, it could not be found. 



* The best method of seeing this experiment, is to take the solution into the open air, where the 

 whole light of a blue sky can fall upon its surface. I have in this way seen the blue line perfectly 

 luminous at that stage of a December twilight when there was not light enough to read by. I con- 

 sider, therefore, the light of the sky as peculiarly susceptible of this species of dispersion. 



