536 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
bited no perceptible sensibility, even when examined by a linear spectrum. However, 
tlie translucency of the substance gave the spectrum a blurred appearance, and the 
fixed lines were not shown so well as on paper. 
Chalk scraped smooth is well adapted, from its fineness, its whiteness and its opa- 
city, for showing the most delicate objects. The finest fixed lines are beautifully seen 
on it, decidedly better than on paper. Its sensibility too, though not absolutely null, 
is much less than that of most kinds of white paper. Indeed, it would be an unne- 
cessary refinement to seek for anything better, were it not that a piece of sufficient 
size might not always be at hand. From what I have seen, I believe that the best 
kind of screen will be obtained by the use of some white inorganic chemical precipi- 
tate, but my experiments in this department have not yet been sufficiently extended 
to authorize me in recommending any particular process. 
191. The object of the observer may however be altogether different, and he may 
wish to extend the spectrum as far as possible, for the purpose of viewing the fixed 
lines belonging to the invisible part beyond the extreme violet, or making experi- 
ments on the invisible rays. For this purpose it would be proper to employ a clear 
and highly sensitive solid or fluid. A weak solution of sulphate or phosphate of 
quinine would do very well, or a weak decoction of the bark of the horse-chestnut 
(no doubt a solution of pure esculine would be better), or an alcoholic solution of the 
seeds of the Datura stramonium. But perhaps the most convenient thing of all would 
be a slab of glass coloured yellow by oxide of uranium. This would be always ready, 
and in point of sensibility the glass does not seem to yield to any of the solutions above 
mentioned, at least so far as relates to those rays which are capable of passing through 
glass*. 
192. In making experiments on the invisible rays, it is well to get rid, as far as 
possible, of the glare arising from the bright part of the spectrum, and therefore a 
clear solid or solution is preferable to an opake screen. If it be desired to show the 
fixed lines in the visible and invisible parts of the spectrum at the same time, a screen 
may be employed consisting of paper washed with a moderately strong solution of 
sulphate of quinine, or an alcoholic solution of stramonium seeds. Turmeric paper 
is not, I think, quite so good for showing the fixed lines of very high refrangibility, 
but is at least equally good for the extreme violet and for the rays a good distance 
further on, especially if it has been washed with a solution of tartaric acid. It is 
likely that many other acids would do as well. Very excellent screens might pro- 
bably be prepared by washing paper with a solution of esculine, or even of the bark of 
the horse-chestnut'l', or by covering pasteboard with yellow uranite reduced to fine 
powder, and made to adhere by a weak solution of pure gum Arabic ; but these I 
have not tried. 
* See note F. 
t See note G. 
