1880.] The Phenomena of Fluorescence. 629 
from the line of simple reflection represents the amount of 
refraction. The difference in degrees of refraction constitutes 
the difference between one colour and another, and one 
shade of colour from that which is akin to it. 
That some rays are bent at a more acute angle than 
others is only another way of saying that they are more 
spread apart, dispersed, or more refrangible than others. 
For example, the yellow part of the speCtrum consists of 
more refrangible rays than the red, and the violet than the 
green, and so on. But beyond each end of this rainbow 
band, or visible speCtrum, there are the invisible rays. 
Those at the violet" end are of very high refrangibility, and 
their motion as ether waves is also of very quick periods, 
while those beyond the red end are of low refrangibility and 
of comparatively slow undulatory periods. Now inasmuch 
as ordinary white light, or even diffused daylight, contains 
within itself those other highly refrangible rays, as well as 
those which are visible, it follows that when this light falls 
on a fluorescent body the substance absorbs or quenches 
those rays, and gives back rays which are of slower periods. 
This, then, is a kind of selective absorption ; consequently 
those highly refrangible rays which, as it were, survive, in 
the course of their egress are evidently retarded within the 
molecules of the sensitive substance, to which molecules 
they impart their motion ; these in their turn and by their 
movements excite or reaCt upon the ether, and cause it to 
vibrate in waves of slower periods than the incident waves, 
which latter may be of any degree of refrangibility. It is 
very difficult to account for the phenomena otherwise. 
Moreover, it was foCind that if the incident light was polar- 
ised, the fluorescent rays from this source showed no traces 
of polarisation ; they are, in fadt, always unpolarised. 
Fluorescent rays were also found to be totally incapable of 
producing the" same effect in a like sensitive body. And 
here it must be remarked that Prof. Stokes’s original paper 
on the subject, entitled “ On the Change of Refrangibility 
of Light ” (“ Phil. Trans.,” 1852), furnishes one of the finest 
examples of inductive reasoning to be found in the literature 
of Physical Science. However, it must be borne in mind 
that some investigators are not satisfied with Stokes’s theory, 
and especially when he laid it down as a law that the light 
emerging from a fluorescent body under excitation is always 
of lower refrangibility than the incident rays. 
Thus Prof. Angstrom, while agreeing with Stokes in some 
respedts, considers that the molecular motions of the 
sensitive substance, on the contrary, by adting on the lu- 
