PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 495 
The red dispersion in the second ethereal solution, though decided, was by no 
means copious. In the case of archil merely diluted with water, it had been so 
slight that its existence might have been considered doubtful. It might be supposed 
that the first solution was not sufficiently concentrated to exhibit the red dispersion, 
in which case the red and green dispersions might have been due to the same sensitive 
principle. But an ethereal extract from dried archil, which was plainly concentrated 
enough, did not exhibit the red dispersion, although it did exhibit the orange and 
green dispersions. None of the sensitive principles appear to constitute the chief 
part of the colouring matter of this dye-stuff. 
68. When some of these ethereal solutions were examined by the third method, 
with a lens of shorter focus than usual, the appearance was very singular. At the 
less refrangible end of the spectrum the incident light was quite inactive ; and then, 
on reaching a certain point, a copious dispersion of orange commenced abruptly. 
This continued with no particular change for some distance further on, when it 
passed abruptly into green. The fourth method showed however that the former 
dispersion continued, and was only masked, in the third method of observation, by a 
new and more powerful dispersion of green which then commenced. And in fact 
when the green-dispersing principle was separated, or partially separated, by water, 
the orange dispersion was seen to continue where before it appeared to have been 
exchanged for green. 
69. I ought here to mention that a similar separation did not take place on the 
addition of water only to an ethereal extract from archil previously dried. The con- 
dition which determined the separation in the first case appeared to be the presence 
of a small quantity of ammonia, which would evaporate on drying the archil. And 
in fact when a small quantity of ammonia was added to the extract from dried archil, 
a partial separation was effected. I do not here enter into the question whether one 
of the sensitive principles may be obtained from the other, whether, for example, a 
chemical combination of the orange-dispersing principle with ammonia might disperse 
a green, or a green with a little orange. A solution containing a mixture of the same 
substance in two different states of chemical combination, both compounds being 
sensitive, is not the less justly regarded as containing two distinct sensitive prin- 
ciples. 
70. The preceding results are mentioned, not for their own sake, but merely for 
the sake of the method of examination employed. The results indeed are so imper- 
fect as to be worthless on their own account. A complete optico-chemical examina- 
tion of archil and litmus would itself alone furnish a subject for research of no small 
extent ; but it belongs rather to chemistry than to general physics. It is quite 
possible that internal dispersion may turn out of importance as a chemical test. The 
dispersing such a tint, and the having the dispersed light produced by light of such 
a refrangibility, form together a double character of so peculiar a nature that it 
enables us, so to speak, to see a sensitive principle in a solution containing many sub- 
MDCCCLII. 3 s 
