490 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
57 . On following the active light through the spectrum, in the direction of 
increasing refrangibility, the dispersion was found to commence with a bright, but 
narrow tail of pure red light, which shot right across the vessel. The light by 
which this tail was produced belonged to the more refrangible part of the extreme 
red band which is transmitted by a moderate thickness of the fluid. The activity 
of the incident light commenced almost abruptly : the same, it will be remembered, 
was the case with the absorbing power of the medium. After the tail of red light 
came the intense absorption band No. 1, where the dispersed light was confined to the 
immediate neighbourhood of the surface by which the active light entered. At this 
place a very bright band of dispersed light was visible on looking at the vessel from 
the outside. In this part of the spectrum the active and the dispersed light were both 
red ; but that dispersion was accompanied by a change of refrangibility, was shown 
by the effect of absorbing media. Thus the long red tail and the bright band adja- 
cent to the surface were differently affected by a blue glass, according as it was held 
in the first or the second position ; and the bright band, though much enfeebled, was 
still plainly visible through a considerable thickness of the fluid, although a stratum 
having a thickness of only a very small fraction of an inch was sufficient to absorb 
the rays by which the band was produced. Although the dispersion continued 
throughout the whole of the visible spectrum and beyond, it was comparatively feeble 
in the brightest part of the spectrum. It became pretty copious again in the neigh- 
bourhood of the dark band No. 4, and remained copious throughout the blue and 
violet. In the green, the dispersed light was red, slightly verging towards orange, 
and in the blue and violet it was red verging a little towards brown. 
58. It may seem superfluous, after what precedes, to bring forward any further 
proof of the reality of a change of refrangibility. Nevertheless the following experi- 
ment, which was in fact performed at an early stage of these researches, may not be 
deemed wholly unworthy of notice, as not involving the use either of absorbing 
media or of false dispersion. 
A small narrow triangle of white paper was stuck on to the outside of the vessel 
containing the leaf-green, in such a manner that its axis was vertical, and its vertex, 
which was uppermost, was situated at the height of the middle of the spectrum. A 
narrow vertical slit was then placed at the distance of the image of the first slit, 
where the fixed lines were formed, and moved sideways till the light immediately 
beside the fixed line G passed through it. The vessel was then placed a few inches 
behind the slit, and moved sideways till the riband-shaped beam of homogeneous 
light, which passed through the second slit, was incident on the vertex of the 
triangle. On looking at the vessel from the front, as nearly as was convenient 
in the direction of the incident light, there appeared a bright vertical bar cor- 
responding to a section of the incident beam. This bar was of two colours, 
namely, red in the upper half, where the light fell on the fluid, and indigo in the 
under half, where it fell on the paper. On refracting the whole system sideways. 
