538 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
carbon was sufficient to absorb all the rays beyond HAl, so that a hollow prism filled 
with this fluid would be useless in experiments on these rays. It should be remarked 
that the sulphuret of carbon employed was not yellow from dissolved sulphur, but 
apparently as colourless as water. 
196. To determine qualitatively the reflecting power of a polished surface with 
respect to the invisible rays of each particular degree of refrangibility, it would be 
sufficient to form a pure spectrum as usual, reflect the rays sideways before they 
come to the focus of the larger lens, place a sensitive medium to receive them, and 
compare the effect with that produced on the same medium when the rays are 
allowed to fall directly upon it. 
Effect of different Flames. 
197 . Want of sunlight proved to be such an impediment to the pursuit of these 
researches that I was induced to try some bright flames, with the view of obtaining 
some convenient substitute. Candle-light is very ill adapted to these experiments. 
The flame of a camphene-larnp proved no better, perhaps rather worse, for it abounds 
so much in rays belonging to the bright part of the spectrum that the glare of the 
light prevents all observation of faint objects ; and the flame does not appear to be 
rich in invisible rays in anything like the proportion in which it is rich in visible 
ones. The flame of nitre burning on wood or charcoal produced a very good effect, 
exhibiting, when the combustion was most vivid, a copious dispersive reflexion in a 
weak solution of sulphate of quinine contained in a bottle held near it. The tint of 
the dispersed light appeared to be not quite the same as that given by daylight, but 
to verge a little towards violet. However, I do not place very strong reliance on the 
judgment of the eye under such circumstances. A still stronger dispersive reflexion 
was produced by a flash of gunpowder. The tint in this case appeared to be the 
same as that seen by daylight. 
198. While engaged in some of these experiments on bright flames, I was surprised 
by discovering the strong effect produced by the flame of a spirit-lamp, the illumi- 
nating power of which is so feeble. When this flame was held close to a bottle con- 
taining sulphate of quinine, a very distinct dispersive reflexion was exhibited. The 
same was the case with several other sensitive solutions. However, the full effect of 
the flame is not thus exhibited, because a considerable portion of the rays which it 
emits is stopped by glass. It is best observed by pouring the solution into an 
open vessel, such as a wine glass or tumbler, holding the flame immediately over it, 
and placing the eye in or very little below the plane of the surfiice. In this way 
nothing is interposed between the flame and the fluid, except an inch or two of air, 
the absorption produced by which, it is presumed, is insensible ; and the plane strata, 
parallel to the surface, into which the illuminated portion of the fluid may be con- 
ceived to be divided, are all projected into lines, whereby the intensity of the blue 
light is materially increased. It is to be observed further, that if the eye be held a 
