482 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
as it had done in the case of a solution of sulphate of quinine. Indeed, it was difficult 
to say how far the decrease in the depth to which the incident rays could be traced, 
by means of the dispersed light which they produced, was due merely to the increasing 
faintness of the light, and how far it indicated a real increase in the absorbing energy 
of the crystal ; whereas in the case of sulphate of quinine the appearance presented 
unequivocally indicated a very rapid increase of absorbing power. 
35. On examining the crystal by the second method, the general appearance was 
the same as in the case of sulphate of quinine, but the beam of falsely dispersed light 
was absent. In addition to the copious beam of deep blue light dispersed by the 
most refrangible rays, there was however a faint beam of red or reddish light dispersed 
by rays of low refrangibility. This beam was too faint to be seen by the third method 
of examination. It will be remembered that the prismatic analysis of the transmitted 
light gave a band of absorption in the red. Another crystal of a pale colour, which 
did not give a similar band of absorption in the red, exhibited nothing but the blue 
beam of dispersed light when examined by the second method. 
36. On examining the crystal by the fourth method, the extreme red proved inac- 
tive. The activity commenced about the most refrangible limit of the red trans- 
mitted by a deep blue glass, when the dispersed light was red, but extremely faint. 
On moving the lens onwards through the spectrum, the dispersed light rapidly 
became brighter, and then died away. When at its brightest, although even then it 
was almost too faint for prismatic examination, it appeared to consist of not quite 
homogeneous light a little lower in refrangibility than the active light. For a con- 
siderable distance further on there was no sensible dispersion produced. The di- 
spersed light became again perceptible when the active light belonged to the greenish 
yellow, or not till the blue, according to the intensity of the incident light. As the 
lens moved on the dispersed light remained faint for a considerable time. It was 
first reddish and then brownish, with a refrangibility answering to its colour. When 
the active light was at H, or thereabouts, the dispersed light rapidly grew much 
brighter, and became of a fine blue. On analysis it was found to consist of rays the 
refrangibility of which ranged within wide limits. The red rays were, however, 
almost wholly wanting, while the rays belonging to the more refrangible part of the 
spectrum resulting from the analysis of the dispersed beam were particularly copious. 
The most refrangible limit of the dispersed light did not quite reach in refrangibility 
the active light. The dispersed light was most copious when the active light belonged 
to the neighbourhood of H. As the lens moved on the dispersed light grew less 
bright, and gradually died away. 
Solution of Guaiacum in Alcohol. 
37 . This is one of the media mentioned by Sir David Brewster, who remarks that 
it “ disperses, by the stratum chiefly near its surface, a beautiful violet light.” 
