PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 483 
When this fluid is examined by the third or fourth method, it is found to exhibit a 
copious internal dispersion, which begins to be conspicuous much lower down in the 
spectrum than in the cases already described. In observing by the third method, 
the true dispersion appeared to commence about the end of the green, the dispersed 
light being reddish-brown. By the fourth method the dispersion could be traced as 
low down as h, the dispersed light being reddish. As the lens moved onwards, 
in a direction from the red to the violet, the more refrangible colours entered in suc- 
cession into the dispersed beam, and it became successively brownish, yellowish, 
greenish, and bluish. In whatever part of the spectrum the lens might be, it was 
found that the most refrangible part of the dispersed beam was of lower refrangibility 
than the active light. This could be easily determined by means of the beam of 
falsely dispersed light, which was always visible so long as the active light belonged 
to the visible part of the spectrum. 
38. With the third arrangement the fixed lines were seen as before by means of 
the dispersed light, but in this fluid they could be seen much lower down in the 
spectrum than in the solution of sulphate of quinine. The group H was seen on a 
greenish ground. About the group I the ground was still greenish, but the dispersed 
light was not very copious. The beautiful violet light mentioned by Sir David 
Brewster is produced only by rays of extremely high refrangibility, and is found to 
extend from the beginning of the group m to the end of the group n, and even further. 
This part of the dispersion is best seen with a rather dilute solution. 
39. In a solution of guaiacum, just as in the solution of sulphate of quinine, the 
absorbing power of the medium increases very rapidly with the refrangibility of the 
light. This is shown by the rapid decrease in the distance from the surface to which 
the dispersed light can be traced. The reason why the violet dispersed light is con- 
fined to a very thin stratum adjacent to the surface by which the light enters, is simply 
that the medium is so nearly opake with regard to the invisible rays beyond the ex- 
treme violet that all such rays are absorbed by the time the light has passed through 
a very thin stratum of the fluid. 
40. If the solution be strong the colour is of considerable depth. In all such cases 
it is necessary to take the precaution, mentioned by Sir David Brewster, of trans- 
mitting the incident beam as near as possible to the upper surface, so as just to graze 
it. The absorption of the medium would otherwise modify the tint of the dispersed 
beam. 
41. The solutions of quinine and guaiacum present a striking contrast with respect 
to the change of tint of the dispersed beam. In the former solution the change is 
but slight, if we except that part of the dispersion which is very faint; whereas in 
the latter, the prismatic colour which makes the nearest match to the composite tint 
of the dispersed beam runs through nearly the entire spectrum, as the refrangibility 
of the active light changes from that of the green rays to that of invisible rays situ- 
ated far beyond the extreme violet. 
