468 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
and then the dispersed light. In the first position the light incident on the fluid 
under examination would be “ epipolized” by transmission through the fluid contained 
in the vessel, and therefore the blue band would be cut off, whereas when the vessel 
was held in front of the eye the blue band would be freely transmitted. Hence the 
effects of the coloured glasses are analogous to, but less striking than, the effect of 
a stratum of the solution of sulphate of quinine in the imaginary experiment above 
described. There is to be sure one important difference in the two cases, namely, 
that in the case of the stratum of fluid the epipolic dispersion which is prevented in 
the fluid under examination is produced near the first surface of the stratum, whereas 
no such dispersion is produced, or at any rate necessarily produced, in the coloured 
glasses. Whatever the reader may think of the results obtained with coloured glasses, 
the next experiment it is presumed will be deemed conclusive. 
10. The board in the window containing the lens having been replaced by a pair 
of boards adapted to form a vertical slit, the sun’s light was reflected horizontally 
through the slit, and transmitted through three Munich prisms placed one after the 
other close to it. A tolerably pure spectrum was thus formed at the distance of some 
feet from the slit. A test tube containing the solution was then placed vertically a 
little beyond the extreme red of the spectrum, and afterwards gradually moved 
horizontally through the colours. Throughout nearly the whole of the visible spec- 
trum the light passed through the fluid as it would have done through so much 
water; but on arriving nearly at the violet extremity a ghost-like gleam of pale blue 
light shot right across the tube. On continuing to move the tube, the blue light at 
firs^t increased in intensity and afterwards gradually died away. It did not however 
cease to appear until the tube had been moved far beyond the violet extremity of the 
spectrum visible on a screen. Before disappearing, the blue light was observed to 
be confined to an excessively thin stratum of fluid adjacent to the surface by which 
the light entered, whereas when it first appeared, namely when the tube was placed a 
little short of the extreme violet, the blue light had extended completely across it. 
It was certainly a curious sight to see the tube instantaneously lighted up when 
plunged into the invisible rays : it was literally darJcness visible. Altogether the phe- 
nomenon had something of an unearthly appearance. 
11. Since the fluid is so intensely opake with regard to rays of extreme refrangi- 
bility, it might be expected, that, though it appears transparent and colourless when 
examined merely by viewing a white object through it, it would yet exhibit a very 
sensible absorbing action with regard to the extreme violet rays when subjected to 
prismatic analysis. To try whether such were really the case, I reflected the sun’s 
light horizontally through a slit, at which was placed a test tube filled with the liquid, 
and analysed the line of light by a prism, the eye being defended by a deep blue glass. 
I was barely able to make out the fixed line H in Plate XXV., that is, the less refrangible 
band of the pair, although in similar circumstances I can generally see about as far 
beyond the more refrangible band as it is beyond H. However, to make the result 
