1853.] OF THE ROYAL INSTITUTION. 263 
nium, a medium which possesses properties similar to those of a 
solution of sulphate of quinine in a still more eminent degree. 
The difference of nature of the illumination produced by a change 
of refrangibility, or “‘ true internal dispersion,” from that due to the 
mere scattering of light, may be shown in a very instructive form 
by placing paper washed with sulphate of quinine, or a screen of 
similar properties, so as to receive a long narrow horizontal spec- 
trum, and refracting this upwards by a prism held to the eye. 
Were the Juminous band formed on the paper due merely to the 
scattering of the incident rays, it ought of course to be thrown 
obliquely upwards; whereas it is actually decomposed by the prism 
into two bands, one ascending obliquely, and consisting of the 
usual colours of the spectrum in their natural order, the other 
running horizontally, and extending far beyond the more refran- 
gible end of the former. Whatever be the screen, the horizontal 
band is always situated below the oblique, since there appears: to be 
no exception to the law, that when the refrangibility of light is 
changed in this manner it is always lowered. 
The general appearance of some highly ‘ sensitive ”’ media in the 
invisible rays was then exhibited by means of the flame of sulphur 
burning in oxygen, a source of these rays which Dr. Faraday, to 
whose valuable assistance the Lecturer was much indebted, had in 
some preliminary trials found very efficacious. The chief media 
used were articles made of glass coloured by uranium, and solutions 
of quinine, of horse-chestnut bark, and of the seeds of the datura 
stramonium. A tall cylindrical jar filled with water showed 
nothing remarkable ; but when a solution of horse-chestnut bark 
was poured in, the descending fluid was strongly luminous. The 
experiment was varied by means of white paper on which words 
had*been written with a pretty strong solution of sulphate of 
quinine, an alcoholic solution of the seeds of the datura stramonium, 
and a purified aqueous solution of horse-chestnut bark. By gas- 
light the letters were invisible ; but by the sulphur light, especially 
when it had been transmitted through a blue glass, which transmits 
a much larger proportion of the invisible than of the visible rays, 
the letters appeared luminous, on a comparatively dark ground. A 
glass vessel containing a thin sheet of a very weak solution of 
chromate of potash allowed the letters to be seen as well, or very 
nearly as well as before, when it was interposed between the eye 
and the paper; but when it was interposed between the flame and 
the paper the letters wholly disappeared,—the medium being opaque 
with respect to the rays which caused the letters to be luminous, 
but transparent with respect to the rays which they emitted. 
It was then remarked what facilities are thus afforded for the 
study of the invisible rays. "When a pure spectrum is once formed, 
itis as easy to determine the mode of absorption of an absorbing me- 
dium with respect to the invisible, as with respect to the visible rays, 
It is sufficient to interpose the medium in the path of the incident 
