PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 505 
terpart on the white part. However, to remove all doubts, I refracted the system in 
the direction of the fixed lines, and then turned the prism round the axis of the eye 
through 90°, when the plane of refraction was situated as before. At first the two 
portions of the line G were of course seen in the same straight line ; and the perfect 
continuity with which, as the prism turned round, the appearance changed into 
what had been first seen, left not the shadow of a doubt as to the reality of the dis- 
location. 
91. The cause of the whole appearance is plain enough. The light coming from 
the illuminated part of the yellow paper consisted, in the neighbourhood of G, of two 
portions ; the first, indigo light, which had been scattered in the ordinary way ; the 
second and larger portion, heterogeneous light having a mean refrangibility a good 
deal less than that of G, which had arisen from homogeneous light of higher refrangi- 
bility. The absence of the first occasioned the faint prolongation of the more refracted 
part of the line G ; the absence of the second gave rise to the less refracted part. 
92. The broad bands H were seen faintly but quite distinctly on the white paper. 
On refracting them sideways by a prism of moderate angle held to the eye, they 
became confused, and tinged with prismatic colours. The confused images of these 
bands, seen in the white and coloured parts, were nearly continuous. It thus appears 
that the visibility of the bands H on the white paper was due to a change of refrangi- 
bility which that substance had produced in violet light of extreme refrangibility. 
93. Effects similar to those produced by paper coloured by tincture of turmeric 
are also produced by turmeric powder, or even by the root merely broken across. 
Notwithstanding the roughness of the latter, the bands H and fixed lines far beyond 
are seen with the utmost facility. 
94. These phenomena are much better observed by covering the slit with a deep 
blue glass, which absorbs all the bright part of the spectrum, while it freely trans- 
mits the violet and invisible rays, which are mainly efficient in this class of pheno- 
mena. In this way fixed lines may be seen on common white paper far beyond H. 
These lines may be seen without the use of the blue glass, by allowing the bright 
colours to pass by the edge of the paper, and receiving on it only the extreme violet 
and invisible rays. 
95. Paper coloured by turmeric having exhibited so well the sensibility of that 
substance, I was induced to try various other washed papers, in fact, papers washed 
with most of the fluids with which I had made experiments. I found almost always 
that sensitive solutions gave rise to sensitive papers, exhibiting a change of refrangi- 
bility of the same character as that shown by the solution. Besides the turmeric 
paper, the two most remarkable were paper washed with a pretty strong solution of 
sulphate of quinine, and paper washed with the extract from the seeds of the Datura 
stramonium. I should here observe, that it was not till long after the time when these 
experiments were made that I was acquainted with the high sensibility of a decoc- 
tion of the bark of the horse-chestnut. The former of the papers just mentioned ex- 
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