PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT.' 519 
the bands seen in an experiment due to the Baron von Wrede. There may, it will 
perhaps be said, have been a fissure parallel to the first surface, so as to separate a 
thin plate ; and the interference of the two streams of light reflected respectively on 
the upper and under surface of this plate may have produced the bands observed. 
But various phenomena attending these bands are irreconcilable with such a suppo- 
sition. Towards the edges of the crystal, where flaws did in fact exist, bands of the 
same nature as Von Wrede’s were actually observed. But these had an appearance 
totally different from that of the others. The dark bands of the interference system 
were more intensely black and better defined than those of the other system, and 
were very variable, depending as they did upon the thickness of the plate by which 
they were formed, whereas the bands belonging to the first system were always the 
same. Besides, were these bands due to interference, there is no reason why they 
should be confined to one region of the spectrum, and that by no means the brightest. 
However, to take away all possible doubts respecting the nature of the bands, I 
detached a small scale from the crystal, and having placed it behind a slit in a beam 
of sunlight condensed by a lens, I analysed the transmitted light by a prism. Were 
the bands really due to absorption, they ought to be more distinct in the transmitted 
light, whereas, were they of the nature of Von Wrede’s bands, they ought to be faint, 
and almost imperceptible. The spectrum of the transmitted light contained however 
four dark bands, which were well defined and intensely black. The whole of the 
spectrum beyond the place of the next band was absorbed, which is the reason why 
four bands only were visible. 
145. The absorption bands of green uranite, though they showed great regularity 
with respect to their positions, did not appear very regular with regard to their in- 
tensities. The second, fifth and sixth seemed to me to be more conspicuous than the 
first, third and fourth. I cannot say for certain whether this ought to be attributed 
to fluctuations in the absorbing power of the medium, or fluctuations in the original 
intensity of the solar spectrum, but I am strongly inclined to prefer the former view. 
146. The intervals between the absorption bands of green uranite were nearly equal 
to the intervals between the bright bands of which the derived spectrum consisted in 
the case of yellow uranite. After having seen both systems, I could not fail to be im- 
pressed with the conviction of a most intimate connexion between the causes of the 
two phenomena, unconnected as at first sight they might appear. The more I examined 
the compounds of uranium, the more this conviction was strengthened in my mind. 
147 . Yellow uranite exhibits a system of absorption bands similar to those of green 
uranite. Nitrate of uranium also shows a similar system. In a solution I have ob- 
served seven of these bands arranged at regular intervals. The first absorption band 
coincided with F, the fifth with G nearly. The absorption bands may also be seen 
by analysing the light transmitted through the crystals. The following arrangement 
exhibited at one view the absorption bands and those due to the light which had 
changed its refrangibility. 
MDCCCLTI. 3 x 
