504 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
degree the property of internal dispersion. It was the observation of Sir John 
Herschel’s already mentioned, which led me to try this medium. But it is by no 
means essential that a sensitive substance should be in solution, or in the state of a 
transparent solid, in order that the change of refrangibility which it produces should 
admit of being established by direct experiment, although of course the mode of 
observation must be changed. 
89. A piece of paper was prepared by pouring some tincture of turmeric on it, and 
allowing it to dry. In this way the part which was deeply coloured by turmeric 
was in juxtaposition with the part which remained white, which was convenient in 
contrasting the effects of the two portions. The sun’s light being reflected hori- 
zontally into a darkened room through a vertical slit, the paper was placed in a 
pure spectrum formed in the usual manner. On the coloured part the fixed lines were 
seen with the utmost facility far beyond the line H, on a yellowish ground. The 
colours too of all the more highly refrangible part of the spectrum were totally 
changed. From the red end, as far as the line F, or thereabouts, there was no mate- 
rial change of colour ; but a little further on a very perceptible reddish tinge came 
on, which was quite decided at F^ G, where it was mixed with the proper colour of 
that part of the spectrum. About Gf H the colour became yellowish. The reality 
of a change of refrangibility was easily proved by refracting the spectrum on the 
screen by a prism applied to the eye. When the refraction took place in a plane 
parallel to the fixed lines, they were seen distinctly throughout the spectrum ; but 
when it took place in a plane perpendicular to the former, the fixed lines in the less 
refrangible part of the spectrum, and as far as F, were distinctly seen ; but in the rest 
of the spectrum they were more or less confused, or even wholly obliterated, accord- 
ing to their original strength, the refracting angle and dispersive power of the prism, 
and its distance from the paper. With a prism of small angle the edges of the broad 
bands H were seen tinged with prismatic colours. 
90. The change of refrangibility was further shown by the following observation. 
The paper was placed in the pure spectrum in such a manner that the line of junc- 
tion of the coloured and uncoloured parts ran lengthways through the spectrum, so 
that the same fixed line was seen partly on the coloured and partly on the uncoloured 
portion. On viewing the whole through a prism of moderate angle applied to the eye, 
and so held as to refract the system in a direction perpendicular to the fixed lines, 
the line F was seen uninterrupted, but G was dislocated, the portion formed on the 
yellow part of the paper being a good deal less refracted than that formed on the 
white. The latter was indeed faintly prolonged into the yellow part of the paper, so 
that on this part G was seen double ; but the image which was by far the more in- 
tense of the two was less refracted than that formed on the white paper. The whole 
appearance was such as to create a strong suspicion of some illusion, as if some other 
gruop of fixed lines formed on the yellow part of the paper had been mistaken for G, 
though certainly no reason appears why such a group should not have had its coun- 
