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 reflection 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 toa 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 epipolic dispersion ;” hut as the dispersing action is not thus limited, 
that conclusion must be incorrect. Sir JoHN HeErRscHeL, 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 Jonn does not mention the distance of the plate from the epipolising 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 Joun 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 
ereen 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 Joun 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 Joun 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. 
* 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. 
I i i tg a Ee 
