610 Scientific Proceedings, Royal Dublin Society. 
change, and, as there is no alteration of refrangibility in this case, 
it must be true of each ray separately. 
In the case of the fluorescent body we must have— 
= —-TNy dy = ey +ey— Ty. 
t.e., the absorbed non-fluorescent rays must he the difference 
between the emitted and reflected ones, and the absorbed and 
reflected fluorescent rays must together make up the whole 
emitted as fluorescent or non-fluorescent. 
We have, besides, that all the fluorescent rays emitted by the 
non-fluorescent body must be either absorbed or reflected by the 
other body, so that— 
By=a, +7 = y+ env. 
Also, all the fluorescent rays emitted by the fluorescent body are 
either absorbed or reflected by the other, so that— 
e=A,+R,=E,, 
and we thus immediately obtain that ¢,,=0, or there can be no 
fluorescence between bodies at the same temperature. 
It seems likely from this that fluorescence depends on the 
amplitude of the incident vibration being much greater than that 
of the corresponding vibration of the molecule of the body, and 
that whatever causes the actual distribution of amplitudes of 
vibrations of the different periods produces its effect less com- 
pletely in the case of fluorescent bodies than of non-fluorescent, 
and less rapidly in the case of phosphorescent than non-phos- 
phorescent bodies. 
