262 NOTICES OF THE MEETINGS [Feb. 18, 
in his mind on this supposition, than the mystery respecting the 
nature of epipolized light vanished ; all the parts of the phenomenon 
fell naturally into their places. So simple did the whole explanation 
become, when once the fundamental hypothesis was admitted, that 
he could not help feeling strongly impressed that jit would turn 
out to be true. Its truth or fallacy was a question easily to be 
decided by experiment ; the experiments were performed, and resulted 
in its complete establishment. 
The Lecturer then described what may be regarded as the funda- 
mental experiment. A beam of sunlight was reflected horizontally 
through a vertical slit into a darkened room, and a pure spectrum 
was formed in the usual manner, namely, by transmitting the light 
through a prism at the distance of several feet from the slit, and 
then through a lens close to the prism. In the actual experiment, 
two or three prisms were used, to produce a greater angular separa- 
tion of the colours. Instead of a screen, there was placed at the 
focus of the lens a vessel containing a solution of sulphate of quinine. 
It was found that the red, orange, &c., in fact, nearly the whole of 
the visible rays, passed through the fluid as if it had been mere 
water. But on arriving about the middle of the violet, the path 
of the rays within the fluid was marked by a sky-blue light, which 
emanated in all directions from the fluid, as if the medium had been 
self-luminous. This blue light continued throughout the region 
of the violet, and far beyond, in the region of the invisible rays. 
The posterior surface of the luminous portion of the fluid marked 
the distance to which the incident rays were able to penetrate into 
the medium before they were exhausted. This distance, which at 
first exceeded the diameter of the vessel, decreased with great 
rapidity, so that in the greater part of the invisible region it 
amounted to only a very small fraction of an inch. The fixed lines 
of the extreme violet, and of the more refrangible invisible rays, 
were exhibited by dark planes interrupting the dispersed light. 
When a small portion of the incident spectrum was isolated, by 
stopping the rest by a screen, and the corresponding beam of blue 
dispersed light was refracted sideways by a prism held to the eye, 
it was found to consist of light having various degrees of refrangi- 
bility, with colour corresponding, the more refrangible rays being 
more abundant than the less refrangible. The nature of epipolized 
light is now evident ; it is nothing but light from which the highly 
refrangible invisible rays have been withdrawn by transmitting 
it through a solution of quinine, and does not differ from light 
from which those rays have been withdrawn by any other means. 
The fundamental experiment, excepting that part of it which 
relates to the analysis of the dispersed light, was then exhibited by 
means of the powerful voltaic battery belonging to the Institution, 
which was applied to the combustion of metals. The rays emana- 
ting from the voltaic arc were applied to form a pure spectrum, 
which was received on a slab of glass coloured by peroxide of ura- 
