518 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
and offers nothing remarkable. The minimum of sensibility occurs at the very same 
place in the spectrum, whether the sensitive medium be a solution of nitrate of ura- 
nium or glass coloured yellow by uranium. 
141. Yellow Uranite . — This mineral, when examined in a linear spectrum, proved 
to be sensitive in an extremely high degree. The derived spectrum consisted, as in the 
case of the glass, of bright bands arranged at regular intervals, but in this case six 
were seen, a band being visible in the faint red at the extremity of the spectrum 
which could not be made out in the case of the glass. 
142. Green Uranite, or Chalcolite. — According to M. Peligot the formula of the 
yellow uranite of Autim is PhO^ CaO, 2(U^ O), 8HO, and the green uranite 
differs from the yellow only in having the lime replaced by oxide of copper*. Yet a 
specimen of green uranite on being examined in a linear spectrum proved totally 
insensible. The primitive spectrum showed however a very remarkable system of 
dark bands depending on the absorption of light by the mineral. In examining 
these bands, the previous prismatic decomposition of the light, so far from being 
necessary, is decidedly inconvenient. It is better to dispense with the prisms alto- 
gether, using only the lens, and placing the mineral so that the image of the slit is 
formed upon it. The bright line thus formed is viewed from a convenient distance 
through a prism, the eye being held out of the direction of regular reflexion. The 
position of any bands which may appear in the spectrum can then be determined by 
means of the fixed lines, which are seen at the same time ; or, if it be desired to see 
the latter more distinctly, it will be sufficient to attach a fragment of paper to the 
mineral or other substance, placing it so that the image of the slit is formed partly 
on the paper and partly on the substance to be examined. I have frequently found 
this mode of observation convenient in examining the absorption of light by opake 
substances. The manner in which the absorption of the medium comes into play in 
this case will be considered in greater detail further on (see Art. 176.). 
143. When green uranite was examined in this manner, it showed a very remark- 
able system of dark bands of absorption. These bands were seven in number, or at 
any rate six, and were arranged with all the regularity of bands of interference. The 
first was situated at about SfF, the second at F ; the middle of the sixth fell a very 
little short of G ; the third, fourth and fifth were arranged at regular intervals be- 
tween the second and sixth ; the seventh was situated about as far beyond the sixth 
as the sixth beyond the fifth. The spectrum was so faint in the region of the seventh 
band as to leave some slight doubts respecting its existence. There would not have 
been light enough to see bands further on. 
144. Uranite is highly lamellar in its structure, from whence it is otherwise called 
uran-mica. The reader may perhaps suppose that the dark bands described in the 
last paragraph were bands of interference, which I had mistaken for bands of absorp- 
tion, and that they were really of the nature of Newton’s rings, or more exactly of 
* Annales de Chimie, tom. v. (1842) p. 46. 
