PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 479 
dispersion which is characterized by the quaquaversus polarization,” the phenomenon 
of false dispersion ceases to be of much interest in an optical point of view; while on 
the other hand the phenomenon of true dispersion, which had always been very 
remarkable, is now calculated to excite a great additional interest. It will be convenient 
to mention here the principal characters by which true and false dispersion may be 
distinguished, although it will be anticipating in some measure the results of obser- 
vations yet to be described. 
26 . In true dispersion the dispersed light has a perfectly continuous appearance. 
In false dispersion, on the other hand, it has generally more or less of a sparkling 
appearance, and on close inspection is either wholly resolved into bright specks, or 
so far resolved as to leave on the mind the impression that if the resolution be not 
complete it is only for want of a sufficient magnifying power. 
In true dispersion the dispersed light is perfectly unpolarized. In false dispersion, 
on the contrary, at a proper inclination the light is almost perfectly polarized in the 
plane of retlexion. 
In false dispersion, which is merely a phenomenon of reflexion, the dispersed light 
has of course the same refrangibility as the incident light. In true dispersion hetero- 
geneous dispersed light arises from a homogeneous beam incident on the body by 
which the dispersion is produced. 
27. In those bodies, whether solid or liquid, which possess in a high degree the 
power of internal dispersion, the colour thence arising may be seen by exposing the 
body to ordinary daylight, looking at it in such a direction that the regularly reflected 
light does not enter the eye, and excluding transmitted light by placing a piece of 
black cloth or velvet behind, or by some similar contrivance. It has been usual to 
speak of the colour so exhibited as displayed by reflexion. As however the cause now 
appears to be so very different from ordinary reflexion, it seems objectionable to 
continue to use that term without qualification, and I shall accordingly speak of the 
phenomenon as dispersive reflexion^. Thus dispersive reflexion is nothing more than 
internal dispersion considered as viewed in a particular way. 
28 . The tint exhibited by dispersive reflexion is modified in a peculiar manner by 
the absorbing power of the medium. In the first place, the light which enters the 
eye in a given direction is made up of portions which have been dispersed by particles 
situated at different distances from the surface at which the light emerges. The word 
particle is here used as synonymous, not with molecule, but with differential element. 
If we consider any particular particle, the light which it sends into the eye has had 
to traverse the medium, first in reaching the particle, and then in proceeding towards 
the eye. On account of the change of refrangibility which takes place in dispersion, 
the effect of the absorption of the medium is different for the two portions of the 
whole path within the medium, so that this effect may be regarded as a function of 
* I confess I do not like this term. I am almost inclined to coin a word, and call the appearance fluorescence, 
from fluor-spar, as the ancilogous term opalescence is derived from the name of a mineral. 
3 Q 
MDCCCLII. 
