520 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
148. The sun’s light was reflected horizontally by a mirror, and condensed by 
passing through a large lens. It was then transmitted through a vessel with parallel 
sides containing a moderately strong ammoniacal solution of a salt of copper. The 
strength of the solution, and the length of the path of the light within it, were such 
as to allow of the transmission of a little green besides the blue and violet. A crystal 
of nitrate of uranium was then attached to a narrow slit, and placed in the blue beam 
which had been transmitted through the solution, the crystal being turned towards 
the incident light. The light coming from the crystal through the slit was then 
viewed from behind, and analysed by a prism. A most remarkable spectrum was 
thus exhibited, consisting from end to end of nothing but bands arranged at regular 
intervals. The interval between consecutive bands appeared to increase gradually 
from the red to the violet, just as is the case with bands of interference. Although 
this interval appeared to alter continuously from one end of the spectrum to the 
other, the entire system of bands was made up of two distinct systems, different in 
appearance, and very different in nature. The less refrangible part of the spectrum, 
where only for the crystal there would have been nothing but darkness, was filled 
with narrow bright bands, due to the light which had changed its refrangibility. 
These bands were much narrower than the dark intervals between them, but they 
were not mere lines containing light of definite refrangibility. The more refrangible 
part of the spectrum was occupied by the system of bands of absorption. The in- 
terval between the most refrangible bright band and the least refrangible dark band 
of absorption appeared to be a very little greater than one band-interval, so that had 
there been one band more of either kind the least refrangible absorption band would 
have been situated immediately above the most refrangible bright band. With strong 
light I think I have seen an additional band of this nature. 
149. Pitchhlende. — This mineral proved to be quite insensible, and exhibited no- 
thing remarkable. 
150. Hydrate of Peroxide of Uranium. — Some crystallized nitrate of uranium was 
exposed to a heat a good deal short of redness, whereby most of the acid was expelled. 
The residue was of a deep brick-red colour, and consisted no doubt chiefly of an- 
hydrous peroxide. It was quite insensible. In order to remove any undecomposed 
nitrate, it was boiled with water, whereby the undecomposed nitrate was dissolved, 
and the peroxide converted into a hydrate. This hydrate, after having been washed 
and dried at the temperature of the air, was of an extremely beautiful yellow colour, 
and was I suppose the hydrate U^O®-l-2HO described in chemical treatises. It was 
tolerably sensitive, in fact for an inorganic substance extremely so, though the sensi- 
bility was much less than that of nitrate of uranium, yellow uranite, or canary glass. 
The derived spectrum consisted as before of separate bright bands. A small portion 
of the powder was attached by water to blotting-paper, and dried before a fire. The 
powder thus obtained on paper was duller than before, and inclined a little more to 
orange, though the colour was not much deeper than that of the former hydrate. 
