PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 475 
changes so gradually and so slightly that the eye glides from one part of the dispersed 
beam to another without noticing any change. 
20. It has been already mentioned that the blue beam of dispersed light seen in a 
solution of sulphate of quinine was produced whether the incident light was polar- 
ized in or perpendicularly to the plane of reflexion, or more properly plane of di- 
spersion, that is, the plane containing the incident ray and that dispersed ray which 
enters the eye. A question naturally presents itself, whether the intensity of the di- 
spersed light is strictly the same in the two cases. By combining a lens of rather 
short focus and a doubly refracting prism with the four prisms, I satisfied myself that 
the difference of intensity, if there were any, was not great, but the experiment pre- 
sented some practical difficulties. However, the result of the following experiment 
appeared to be as decisive as a negative result could well be. 
The arrangement being the same as in the third method, but the lens in front of 
the prisms having a focal length of only 6‘5 inches, the incident light was polarized 
in a vertical plane previously to passing through the slit, by transmission through a 
pile of plates. The two beams of light were seen as usual in the fluid, namely, the 
blue beam due to internal dispersion, and the fainter coloured beam due to motes. 
The former of these, which was quite separate from the latter, exhibited the principal 
fixed lines belonging to the highly refrangible part of the spectrum. A plate of 
selenite was then interposed immediately in front of the vessel, so as to modify the 
polarization of the light entering the fluid. This plate was obtained by an irregular 
natural cleavage, and was cemented with Canada balsam between two discs of glass. 
When examined by polarized light it exhibited a succession of beautiful and varied 
tints, according to the various thicknesses of the different parts. Now u^hen the 
plate was moved about in front of the vessel, without altering its perpendicularity to 
the incident light, different portions of the beam due to motes were observed to dis- 
appear and reappear, or at least to become faint and then bright again, so that a 
person ignorant of the cause, and not looking at the disc, might have supposed that 
the observer had been holding in front of the vessel a piece of dirty glass, having the 
dirt laid on in patches ; but in whatever manner the disc was moved in its own plane 
without rotation, or turned round an axis perpendicular to its plane, not the slightest 
perceptible change was produced in any part of the blue beam. 
Explanation of Terms. 
21. In all the experiments described in this paper in which a spectrum was formed 
for the sake of examining the separate action of portions of light of different refran- 
gibilities, the length of the spectrum was horizontal, so that the fixed lines were 
vertical. Nevertheless it will be convenient, for the sake of shortness, to use the 
prepositions above and below to signify respectively on the more refrangible side of and 
on the less refrangible side of. 
The principal fixed lines of the visible spectrum will be denoted by letters in ac- 
