150 
MR. W. CROOKES OK THE ILLUMINATION OF LIKES OF 
that the phosphorescence of the glass might possibly be excited, as we have hitherto 
known phosphorescence to he excited, by radiation, instead of being excited by the 
actual impact of the individual molecules on the molecules of the glass. The difficulty 
of this supposition was, where could the radiation come from ? The sharpness of 
the shadows preclude the idea that it comes from the immediate neighbourhood of 
the negative electrode, which is far from being a mere point. It might, however, 
be supposed that the incandescent molecules were projected with such enormous 
velocity from the negative, that the space around it was kept comparatively 
clear; that there were comparatively few molecules within this space, but that, adja¬ 
cent to the glass, there was a thin layer of them densely packed. Thus, if the 
molecules be thought of as lamps giving out, not the green light observed, but the 
invisible ultra-violet radiation which is capable of exciting the green phosphorescence, 
it is conceivable that the absence of phosphorescence within the sharply-bounded 
shadows might be accounted for by the absence of the thin overlying phosphorogenic 
stratum. 
PHOSPHORESCEKCE OF THIK FILMS. 
533. An experiment was suggested by Professor Stokes which seemed calculated to 
be decisive, or almost decisive, between these different views. If the phosphorescence 
were really produced by impact, it must be confined to a stratum of almost infinitesimal 
thinness, but if it were produced by radiation from a thin layer of densely-packed 
gaseous molecules, glowing with phosphorogenic light, the fluorescence should extend 
a measurable distance into the glass; and if an excessively thin film of the glass were 
taken (thin enough to show the colours of thin plates), and placed in front of a thicker 
plate of phosphorescent glass, the full amount of fluorescence should not be excited on 
it, but some of the phosphorogenic rays would be likely to pass through, and the thin 
film should not cast a black shadow. By taking very thin films of different glasses, and 
also thin plates of quartz, the result would be more conclusively shown, for it is not 
likely that thin layers of all these media possess a metallic opacity to the phospho¬ 
rogenic rays. 
534. To experiment on these points, an apparatus was made in the following 
Fig. 14. 
mannerIn the centre of a tube, furnished with flat aluminium terminals, cl, e, 
is supported a plate of uranium glass, a. About half a millimetre on one side is 
a film of uranium glass, b, and on the other side a similar film of German glass, c. 
The two films are thin enough to show brilliant colours of thin plates when viewed by 
