PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OP LIGHT. 387 
244. In the observation, the first medium is placed resting on the shelf so as to 
cover the hole ; the object is placed on the shelf immediately in front of the hole ; the 
second medium is held anywhere between the eyes and the object. As it is not 
possible to obtain media which are strictly complementary, it will happen that a 
certain quantity of light is capable of passing through both media. This might no 
doubt be greatly reduced by increasing the absorbing power of the media, but it is 
by no means advisable to do so to any great extent, because it is important that the 
second medium should transmit as many as possible of the rays which are of such 
refrangibilities as to be stopped by the first. Accordingly, it might sometimes be 
doubtful whether the illumination perceived on the object were due merely to 
scattered light which had passed through both media, or to really “ degraded ” light*. 
To remove all doubt, it is generally sufficient to transfer the second medium from 
before the eyes to the front of the hole. The light merely scattered by the object 
will necessarily be the same as before, if the room be free from stray light ; and even if 
there be a little stray light, the illumination, so far as it is concerned, will be increased 
instead of diminished ; whereas if the illumination previously observed were due to 
fluorescence, and the media were properly chosen, the object which before was 
luminous will now be comparatively dark. 
Sometimes, in the case of substances which have only a low degree of sensibility, 
it is better to leave the second medium in front of the eyes, and use a third medium, 
which is held alternately in the path of the rays incident on the object and between 
the object and the eyes. Such a medium, though not at all necessary, may be used 
* This term, which was suggested to me by my friend Prof. Thomson, appears to me highly significant. The 
expression degradation of light might be substituted with advantage for true internal dispersion to designate the 
general phenomenon ; but it is perhaps a little too wide in its signification, and might be taken to include 
phosphorescence (if indeed in this case the refrangibility be really always lowered), as well as the emission of 
non-luminous radiant heat by a body which had been exposed to the red rays of the spectrum. As to the term 
internal dispersion, though I employed it, following Sir David Brewster, I confess I never liked it. It seems 
especially awkward when applied to a washed paper or dyed cloth ; it was adopted at a time when the pheno- 
menon was confounded with opalescence ; and, so far as it implies theoretical notions at all, it seems rather to 
point to a theory now no longer tenable : I allude to the theory of suspended particles. Indeed, this theory, 
as it seems to me, ceased to be tenable as soon as Sir John Herschel had discovered the peculiar analysis 
of light connected with epipolic dispersion, and Sir David Brewster had connected the phenomenon with 
internal dispersion, so far at least as the common appearance of a continuous and coloured dispersed beam 
formed a connexion. The expression dependent emission is awkward, but would be significant, because the light 
is emitted in the manner of self-luminous bodies, but only in dependence upon the active rays, and so long 
as the body is under their influence. In this respect the phenomenon differs notably from phosphorescence. It 
is quite conceivable that a continuous transition may hereafter be traced by experiment from the one phenomenon 
to the other, but no such transition has yet been traced, nor is it by any means certain that the phenomena are 
not radically distinct. On this account it would, I conceive, be highly objectionable to call true internal disper- 
sion phosphorescence. In my former paper I suggested the texm fluorescence , to denote the general appearance 
of a solution of sulphate of quinine and similar media. I have been encouraged to give this expression a wider 
signification, and henceforth, instead of true internal dispersion, I intend to use the term fluorescence, which 
is a single word not implying the adoption of any theory. 
