PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 491 
through a prism of moderate angle applied to the eye, the objects appeared in the 
following order as regards refrangibility. First came the upper half of the bright 
bar, which was only a very little widened by refraction, so that it consisted of red 
light which was approximately homogeneous. Next came the triangle, with its 
vertex a little rounded, and its edges tinged with prismatic colours. The vertex, 
which had formerly coincided with the bright bar, now lay a little to one side of its 
upper half. The triangle was of course seen by means of the diffused light of the 
room, which was not perfectly dark, and therefore its refrangibility must have cor- 
responded to the brightest part of the spectrum, or nearly so. Lastly came the 
under half of the bright bar, which was much more refracted than the triangle, so 
as to be shifted almost completely off it. The paper triangle was far too close to 
the first surface of the fluid to allow of attributing the dislocation of the bright bar 
to any error depending upon parallax ; but to prevent all possible doubts on this 
score, 1 took care to refract the system both right and left, and the result was the 
same in the two cases. The conclusion is therefore inevitable, that the indigo light 
which had changed its colour by dispersion from leaf-green had changed its refran- 
gibility at the same time. 
59. In viewing a solution of leaf-green in a pure spectrum, I noticed a phenomenon 
which further indicates the close connexion which seems to exist between the 
absorption and internal dispersion of this fluid. On holding the eye vertically over 
the fluid, and looking down at the dispersed light through a red glass, I observed 
five minima of illumination, having for the most part the shape of teeth with their 
bases situated at the surface by which the light entered, and their points turned 
inwards. These minima occupied positions intermediate between the bands of 
absorption, so far at least as the positions of the latter were indicated by dark teeth 
pointing in the contrary direction. The first minimum was situated a little beyond 
the intense absorption band No. 1, and corresponded in position to the bright band 
No. 2. The second was situated a little further on. The maximum intervening 
between this and the third was but slight, so that the second and third together 
formed pretty nearly one broad minimum. The third and fourth were situated one 
at each side of the dark band No. 4, so as to correspond in position to the bright 
bands Nos. 4 and 5. The fifth was situated a little way beyond the bright band 
No. 5. This last minimum was not tooth-shaped, inasmuch as it occurred at a part 
of the spectrum where the dispersed light was almost confined to the surface of the 
fluid. These minima are best seen when the solution is rather weak. They may be 
perceived without using a red glass, though not so easily as with its assistance. 
With a stronger solution it was observed that the first minimum ran obliquely into 
the dark tooth corresponding to the absorption band No. 1. 
60. The reason of the occurrence of these minima appears to be simply this, that 
the more copiously dispersed light is produced, the more rapidly the incident light 
is used up in producing it, so that minima of activity correspond to points of the 
