PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 489 
band increased in width, and when the red beam was almost absorbed, the part that 
was left consisted of two cones of red, one at each side of the dark band, which by 
this time had become broad. The whole appearance seemed to indicate that the 
bright red beam of dispersed light had a very intimate connexion with the intense 
absorption band No. 1. 
54. Among coloured glasses, there is one combination which produces a very 
striking effect. When a deep blue glass is placed in the first position, the dispersed 
light, if the solution be at all strong, is confined to a very thin stratum adjacent to 
the surface, and is best seen by placing the vessel so that the surface of the fluid at 
which the light enters is situated at a little distance on either side of the focus of 
the lens, when there is seen a bright circle of a most beautiful crimson colour. It 
might be supposed that the red of which this circle mainly consists was nothing 
but the extreme red transmitted by the blue glass. But it is readily shown that 
such is not the case. For in the first place, the fluid transmits pretty freely the red 
transmitted by the blue glass, whereas the red light found in this circle is almost 
confined to the surface of the fluid. Again, it was found that a pale brown glass, 
which transmitted freely the extreme red, almost entirely cut off the bright circle, 
when placed in the first position without removing the blue glass, although it freely 
transmitted it when placed in the second position. It appears, therefore, that the 
bright circle is due, not to the red, but to the highly refrangible rays transmitted by 
the blue glass. 
55. When a solution of leaf-green was examined by the third method, the appear- 
ance as seen from the outside was very singular. The fixed lines in all the more 
refrangible part of the spectrum were seen as interruptions in a bright red ground 
verging to crimson. The beauty and purity of the tint, and the strange ct ntrast 
which it presented to the colours belonging to that part of the spectrum, were very 
striking. About H the tint began to verge towards brown, and the fixed lines 
beyond H were seen on a brownish red ground. That the ground on which the 
fixed lines of somewhat less refrangibility were seen was rather crimson than red, 
arose, no doubt, from the mixture of a little blue or violet light due to false disper- 
sion, and to the scattering which took place at the surface of the glass. 
56. On looking down from above, the places of the more conspicuous bands of 
absorption were indicated by dark teeth, with their points turned towards the inci- 
dent light, interrupting the dispersed light. It is to be understood that the light 
was transmitted as close as possible to the upper surface, so that the absorption by 
which these teeth were formed took place before dispersion. In this way the places 
of the absorption bands Nos. 1, 2 and 4, were perfectly evident. No. 3, it will be 
remembered, was by no means conspicuous. When the solution is of convenient 
strength, the absorption is so rapid beyond the bright band No. 5, that the disper- 
sion is confined to a thin stratum close to the surface by which the light enters, and 
therefore no dark tooth would be seen corresponding to the dark band No. 5. 
3 R 2 
