PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITV OF LIGHT. 535 
that of Art. 176, we find for the intensity I' of the dispersed light which enters the eye 
^ j + s 
lo being the intensity of the incident light. Since a sensitive fluid is in general coloured, 
and the dispersed light is in general heterogeneous, s will in general be different for 
the different portions into which the dispersed light would be decomposed by a prism. 
However, if the fluid be colourless, or all but colourless, as is the case with a solution 
of sulphate of quinine, s will be insensible, so that I' will be proportional simply to 
rq~\ Hence from the observed variations in I', arising from variations in the strength 
of the solution, we may infer the corresponding variations in rq~\ 
If, then, we represent by the ordinate of a curve the ratio of the quantity of light 
given out to the quantity of light absorbed by a given number of active molecules, 
the abscissa being the ratio of the quantity of diluting fluid to the quantity of the 
sensitive substance in solution, it appears that the curve will be concave towards the 
axis of the abscissae, and will have an asymptote parallel to that axis. 
On the Choice of a Screen. 
189. We have seen that white paper, the substance commonly employed as a 
screen on which to receive the spectrum, gives back with a changed refrangibility a 
portion of the light incident upon it. This might in some cases lead an observer 
not aware of the circumstance to erroneous conclusions. ISince the colour of dispersed 
light depends upon its refrangibility, which is different from that of the active light, 
the colours of a spectrum received on white paper must be somewhat modified. In 
truth the intensity of the light dispersed is so small compared with the intensity of 
the light scattered, that the modification is quite insensible except in the extreme 
violet. But beyond the extreme violet the spectrum seems to be prolonged with a 
sort of greenish gray tint, which belongs neither to that nor to any other part of the 
true spectrum. In experiments on absorption, if instead of receiving the light 
directly into the eye it be found convenient to form a pure spectrum on a screen of 
white paper, then, if the absorbing medium be placed in the path of the incident 
light, the scattered light forming any part of the spectrum cannot be cut off or 
weakened without at the same time cutting off or weakening the dispersed light coming 
from the same part of the screen. But if the absorbing medium be held in front of 
the eye, its effect on the spectrum will sometimes be very sensibly different from what 
it would be were the screen to send back none but scattered light. 
It is true that the quantity of light dispersed by white paper is so small that this 
substance may very well continue to be used*as a screen, without any danger of the 
observer’s being deceived, if only he be aware of the fact of dispersion, so as to be on 
his guard. Still, it is not unreasonable to seek for a substitute for paper, which may 
be free from the same objection. 
190. A porcelain tablet appeared to be unexceptionable in this respect, for it exlii- 
MDCCCLII. 3 z 
