472 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
which allowed a direct comparison of the two images, it offered no traces of polar- 
ization. It was produced by light polarized in a vertical or horizontal plane as well 
as by common light, and in that case, as well as in the former, manifested no traces 
of polarization*. 
The short distance that the more refrangible rays were able to penetrate into the 
fluid might readily be perceived in this experiment, but the second method of obser- 
vation was not adapted to bring out this part of the phenomenon. 
16 . On examining the fluid by the third method, the result was very striking, 
although of course only what might have been anticipated. The principal fixed lines 
of the violet, and of the chemical parts of the spectrum beyond, were seen with 
beautiful distinctness as dark planes interrupting an otherwise perfectly continuous 
mass of blue light. To see any particular fixed line with most distinctness, it was of 
course necessary to hold the eye in the corresponding plane, when the dark plane 
was foreshortened into a dark line. From the red end of the spectrum, as far as the 
line G, or thereabouts, the light passed freely through the fluid, or at least was only 
reflected here and there from motes held in mechanical suspension. About G the 
dispersion just commenced to be sensible, and there were traces of that line seen as a 
dark plane interrupting a mass of continuous but excessively faint light. For some 
distance further on the dispersed light remained so faint that it might have been 
passed over if not specially looked for. It was about half-way between G and H, or 
a little before, that it first became so strong as to arrest attention, and a little further 
on it became very conspicuous, the tint meanwhile changing to a pale sky-blue. The 
light was very copious about the two broad bands of the group H, and for some 
distance from H towards G. Some of the fixed lines less refrangible than H were 
very plain, and beyond H a good number were visible, which will presently be further 
described. The whole system of fixed lines thus visible as interruptions in the di- 
spersed light had a resolvable appearance ; but with a very narrow slit and a lens of 
long focus at the prisms the light would have been too faint for convenient obser- 
vation. 
The dispersed light about G, and for some distance further on, was so very faint 
that I might have overlooked it had it not arrested my attention when observing by 
the fourth method ; indeed, I have sometimes specially looked for it in the third 
arrangement without having been able to see it. Practically speaking, the dispersion 
might be said to commence about half-way between G and H. 
* These two results, namely, that the blue beam which constitutes the greater part of the light dispersed 
by a solution of sulphate of quinine is unpolarized, or according to his expression possesses a qtiaquaversus 
polarization, and that that still remains the case when the incident light is polarized, have been already 
announced by Sir David Brewster, who appears to have been led to attend to the polarization of the light 
from Sir John Herschel’s observation, that the blue light arising from epipolic dispersion in a solution of sul- 
phate of quinine was unpolarized. It seemed important however to repeat the observation on the blue beam 
obtained in a state of isolation. 
