506 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
hibited the fixed lines of the invisible rays on a blue, and the latter on a green 
ground. The dispersion produced by the quinine paper was not exhibited so early in the 
spectrum as in the case of turmeric, nor was it so copious in the extreme violet rays, 
and for some distance further on, but the quinine paper seemed superior to the other 
for showing the fixed lines of extreme refrangibility. With the turmeric paper the 
group n was plain enough, but with the quinine paper I have seen some fixed lines of 
the group p. The stramonium paper was, on the whole, I think superior to the qui- 
nine paper in point of the copiousness of the dispersed light, but seemed hardly equal 
to it for showing the fixed lines of extreme refrangibility. However, it is likely that 
paper washed with a solution of the sensitive principle in a state of purity would have 
been quite equal to the quinine paper in this respect. 
96. A washed paper is a little more convenient for use than a solution, but, as 
might be expected, it does not show the fixed lines with quite as much delicacy, nor 
is it quite so good for tracing the spectrum to the utmost limits to which it can be 
traced with the substance employed. 
97 . The sensibility of fresh leaf-green could not be made out on a washed paper 
by this mode of observation, but the sensibility of the substance extracted by alcohol 
from black tea, from which the brown colouring matter had been removed by hot 
water, was plainly exhibited by the redness which it produced in the highly refran- 
gible part of the spectrum. 
98. Paper washed with a solution of guaiacum seemed an exception to the general 
rule ; but this is not to be wondered at, since a paper prepared in this manner is 
turned green when exposed to the light, and it is difficult to prevent some degree of 
discoloration. That the fluid state is not essential to the exhibition of the sensibility 
of this substance, was however plainly shown by the high degree of sensibility of the 
solid resin from which the solution was made. In this case the bands H were seen 
on a greenish ground. The dispersion of a fine blue light under the influence of rays 
of still higher refrangibility was hardly, or not at all, exhibited by the solid resin. 
99. Shell-lac, common resin, glue, are all highly sensitive. The ground on which 
the fixed lines in the neighbourhood of H are seen is brown in the case of shell-lac, 
and greenish in the case of resin and glue. The sensibility of glue is evidently not 
due to gelatine, for isinglass is almost, if not quite, insensible. These are merely a 
few instances of sensibility : I shall defer further mention of the subject till I have 
described a better mode of observation. I will merely observe for the present, that 
several washed papers proved not greatly inferior to turmeric paper for showing the 
fixed lines about and beyond H. 
Effect of refracting a Narrow Spectrum in a Vertical Plane. 
100. In the arrangement last described, when a short slit is used, the spectrum 
received on the washed paper or other substance is of course narrow, so that the 
fixed lines formed on the paper are but short, and may roughly be regarded as mere 
