PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 503 
the fluid which is under the influence of the active light, and emanates apparently 
in all directions alike. I have not attempted to determine experimentally whether 
the intensity is strictly the same in all directions. The experiment would be very 
difficult, especially for directions nearly coinciding with that of the active light, 
because in that case the light which was really due to internal dispersion would be 
mixed up with the glare which is always found in the neighbourhood of light of 
dazzling brightness. However, I have seen nothing which led me to suppose that 
the intensity was different in different directions. We may express the results of 
observation extremely well, by saying that the fluid or solid medium is self-luminous 
so long as it is under the influence of the active light. 
Accordingly, when a bright object, such as the sky, or the flame of a candle, is 
viewed through a highly sensitive fluid, the regularly transmitted light is accompa- 
nied by some side light due to internal dispersion. The latter, however, emanating 
in all directions alike from the influenced particles, is too faint, when contrasted 
with the regularly transmitted light, to make any sensible impression on the eye. 
But when a fluid, itself insensible, holds in suspension a great number of solid 
particles of finite size, the light reflected from such particles is reinforced, in direc- 
tions nearly coinciding with that of the incident light, by a great quantity of diffracted 
light, so that a bright object viewed through such a fluid is surrounded by a sort of 
nebulous haze, giving the fluid a milky appearance. 
Washed Papers. 
8/. In a paper “ On the Action of the Rays of the Solar Spectrum on Vegetable 
Colours,” Sir John Herschel mentions a peculiarity which he had observed in 
paper washed with tincture of turmeric, which consists in its being illuminated, 
when a pure spectrum is thrown on it, to a much greater distance at the violet end 
than is the case with mere white paper*. This phenomenon was attributed by Sir 
John to a peculiarity in its reflecting power, and was considered as a proof of the 
visibility of the ultra-violet rays. The colour of the prolongation of the spectrum 
was yellowish green. Sir John appears to have been in doubt whether the greenish 
yellow colour was to be attributed to the mixture of the true colour of the ultra- 
violet rays with the yellow of the paper due to diffused light, or to the real colour of 
the ultra-violet rays themselves, which on that supposition Mmuld have been incor- 
rectly termed “ lavender.” 
88. The fact of the change of refrangibility of light having been established, there 
could be little doubt that the true cause of the extraordinary prolongation of the 
spectrum on paper washed with tincture of turmeric, was very different from what 
Sir John Herschel had supposed, and that it was due to a change of refrangibility 
in the incident light, which was produced by the medium in a solid state. Tincture 
of turmeric has already been mentioned as a medium which possesses in a high 
* Philosophical Transactions for 1842, p. 194. 
3 T 
MDCCCLII. 
