488 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
and there is left the red band No. 1, and a double green band, consisting of the 
bright bands Nos. 4 and 5, separated by the dark band No. 4, which by this time 
has come out. In modified leaf-green, the dark bands Nos. 4 and 5 are much more 
conspicuous than in the green fluid, but No. 3 is wanting, or all but wanting. With 
a thickness by which the absorption is well developed, the conspicuous bright bands 
are in this case Nos. 1 and 3, and next to them No. 2, whereas in the green fluid 
Nos. 2 and 3 were quickly absorbed, or at least the whole of No. 2, and the greater 
part of No. 3. 
51. It seems worthy of remark, that, especially in the case of the green fluid, the 
absorbing power alters with the refrangibility of the light at a very different rate on 
the two sides of the intense dark band No. 1. This might be inferred from the order 
in which the bright bands disappear ; but it was rendered visible to the eye by the 
following easy experiment. A narrow test tube was partly filled with a solution of 
leaf-green, and then a few drops of alcohol were added, which remained at the top, 
and there diluted the solution. The tube was then held before a candle, and the 
linear image of the flame was viewed through a prism. In the under part the dark 
band No. 1 was broad, the bright band No. 2 being narrow, and almost obliterated, 
but in the upper part the dark band No. 1 was very narrow. Now on tracing 
upwards the sides of this dark band, it was found that the less refrangible side was 
almost straight, and the diminution in the breadth of the band was produced by the 
encroachment of the bright band No. 2. Speaking approximately, we may say that 
in proceeding from the extreme red onwards, at a certain point of the spectrum the 
fluid passes abruptly from transparent to opake, and then gradually becomes almos 
transparent again. 
52. It may here be remarked, that although the absorption produced by leaf-green 
is best studied in a solution, its leading characters may be observed very well by 
merely placing a green leaf behind a slit, as near as possible to the flame of a candle, 
and then viewing the slit through a prism. 
53. After this digression relating to the absorption of leaf-green, it is time to 
come to its internal dispersion. And first, when a cone of white light coming from 
the sun is admitted horizontally into the fluid, as close as possible to its upper 
surface, and the beautiful red beam of dispersed light is analysed by a prism, the 
spectrum is found to consist of a bright red band of a certain breadth, followed by 
a dark interval, and then a much broader green band not near so brilliant. There 
is usually but little false dispersion, and what there is may be almost entirely got 
rid of by analysing the beam by a Nicol’s prism, so as to view it by light polarized 
in a plane perpendicular to the plane of dispersion. Now on raising the vessel with- 
out removing the prism from the eye, it was found that a dark band, which was in 
fact the absorption band No. 1, appeared almost exactly in the middle of the bright 
red band. On continuing to raise the vessel, so as to make the dispersed rays pass 
through a still greater thickness of the medium before reaching the eye, the dark 
