PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 509 
great facility ; but I was not able to make out fixed lines beyond the end of the group 
/, that is, about the end of Fraunhofer’s map. However, the eyes of different 
individuals may differ much in their power of being affected by the highly refrangible 
rays, ft must be confessed, that on looking in the direction of the prisms, a good 
deal of blue light was seen, consisting of light which had been scattered at the 
surfaees of the prisms and lens. This light, though far from dazzling, was sufficient 
to prevent the eye from seeing excessively faint objects, even though they might be 
well defined. For want of a heliostat, I did not attempt an experiment I was medi- 
tating for securing a more perfect isolation of the ultra-violet rays*. 
However, it seems to me to be a point of small importance, so far as regards its 
bearing on other physical questions, whether the illuminating power of these rays is 
absolutely null or only excessively feeble. It is quite certain, that if not absolutely null, 
their illuminating power is at least utterly disproportionate to the effect which they pro- 
duce in the phenomena to which the present paper relates, and indeed that is true 
even of the violet rays. By illuminating power, I mean of course, power of producing 
the sensation of light when received directly into the eye; for by giving rise to light 
of lower refrangibility, they are able to illuminate strongly an object on which they 
fall. 
Mode of Observation specially applicable to Opake Bodies. 
106. In some of the experiments already described, the change of refrangibility 
was exhibited, which was produced by washed papers and solid bodies. There exists, 
however, a mode of observation far preferable to those which have already been 
explained as applicable to such cases, and which may even in some instances be 
employed with advantage in the examination of transparent bodies. In the experi- 
ment described in Art. 100, the primitive spectrum is pure, but the derived spectrum 
impure, on account of the finite length of the slit. Were the slit reduced to a point, 
it is true that the derived spectrum would become pure like the primitive, but then 
the quantity of light would be so small that the primary spectrum would hardly 
bear prismatic analysis. It is well, once for all, to examine a few sensitive opake 
substances in a very pure spectrum, because then the exhibition of fixed lines running 
across the colours in the derived spectrum removes even the shadow of a doubt as to 
the reality of the change of refrangibility of the incident light. Besides this, the 
only theoretical advantage in having the primitive spectrum very pure is, that it 
might be expected to enable us to detect any very rapid fluctuations in the colour or 
intensity bf the dispersed light. Of course, I am now speaking only with reference 
to experiments in which the observer is employing the spectrum to examine some 
substance, not employing the substance to examine the spectrum. But practically, 
I have not found any advantage on this account ; for abrupt, or almost abrupt 
changes in the colour or intensity of the dispersed light hardly ever, if ever, occur, 
* See note B. 
