PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 523 
similar dark band, but not quite so intense, occurs in the green : beyond the green 
there is usually but little light seen. As the absorption progresses the first dark band 
invades all the space from the red to the green, and the spectrum consists of an iso- 
lated red band and a green band divided into two. In its mode of absorption, the 
medium has a strong general resemblance to chlorophyll. The green due to copper 
or to chromium shows nothing remarkable when viewed through a prism, and could 
not possibly be confounded with the green due to protoxide of uranium. The absorp- 
tion bands due to this oxide are not completely brought out till the bead is cold. 
161. Uranium produces the same effects with borax as with microcosmic salt, but 
they are less distinct, or at least less easily produced. 
162. When the uranium contained in a bead of microcosmic salt is thoroughly 
oxidized, and the bead is gently heated, so as just to be self-luminous, the light 
which it gives out is not red, like that of most substances at a low heat, but green, 
or rather greenish white. 
163. Solutions of protoxide of uranium have a very remarkable effect on the spec- 
trum, resembling more or less that of a bead of microcosmic salt coloured green by 
uranium. Of course the absorption can be observed much better by means of a solu- 
tion than by a mere bead. I have observed several bands of absorption in such 
solutions, but the cases which I have hitherto examined are too few to justify me in 
entering into detail. Besides, the absorption bands due to protoxide of uranium do 
not belong properly to my subject, the compounds of this oxide, so far as I have ex- 
amined, being insensible. 
Appearance of highly Sensitive Media in a Seam from which the Visible Rays are 
nearly excluded. 
164. When a large beam of sunlight is reflected horizontally into a darkened 
room, and transmitted through an absorbing medium, placed in the window, of such 
a nature as to let pass only the feebly illuminating rays of high refrangibility and the 
invisible rays beyond, various sensitive media have a very strange and unnatural 
appearance when placed in the beam, on account of the peculiar softness of the di- 
spersed light with which the media appear as it were self-luminous, and the almost 
entire absence of strong light reflected from convexities. Among substances emi- 
nently proper for this experiment, may be mentioned a solution of the bark of the 
horse-chestnut, or of sulphate of quinine, or of stramonium seeds, a decoction of 
madder in a solution of alum, and above all, ornamental articles of canary glass. 
The appearance of a specimen of yellow uranite was curiously altered by this mode 
of examination. By daylight the mineral appeared much of the same colour as the 
stone in which it was imbedded, but when placed in a beam such as that above men- 
tioned the uranite was strongly luminous, while the stone remained dark. 
