368          ON THE  CHANGE  OF  REFRANGIBILITY  OF  LIGHT.
of refrangibility. Then by a very simple calculation similar to that of Art. 176, we find for the intensity /' of the dispersed light which enters the eye
I_0 being the intensity of the incident light. Since a sensitive fluid is in general coloured, and the dispersed light is in general heterogeneous, 5 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, ,v will be insensible, so that 1' will be proportional simply to rtf\ Hence from the observed variations in /', arising from variations in the strength of the solution, we may infer the corresponding variations in rf^l.
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 bein»' 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 abscissa*, and will have an asymptote parallel to that axis,
On the Choice of (t> 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 iu some cases lead an observer not. aware of the circumstance to erroneous conclusions. Since (.he 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