ON ENERGY OF EONTGEN AND BECQUEREL RAYS, ETC. 
35 
Efficiency of a Fluorescent Screen as a Source of Light. 
Experiments were made to determine the efficiency of a fluorescent screen as a 
transformer of Rontgen radiation into visible light. 
Photometric observations of the light emitted by a fluorescent screen, excited by 
X rays, have been made by A. Moffat,* who deduced the energy of tlie rays, by 
assuming that the coefficient of transformation of the energy into visible light was 
4 per cent., the value found by E. Wiedemann! for the transformation of radiant 
energy into luminescence. 
It was not the object of this investigation to make a complete photometric com¬ 
parison, but to deduce an ajiproximate coefficient of transformation for a definite 
experimental arrangement which could readily be reproduced in practice. For this 
purpose a piece of fluorescent screen was placed over one of the diffusive surfaces ot 
a Lummer-Brodhun screen, and the diffused light of the screen compared with the 
diffused light of the amyl lamp in the usual manner. The ratio of the square of the 
distance of the screen from the bulb to the square of the distance of the lamj) was 
taken as the ratio of the intensities of the light emitted by the X-ray bulb and lamp. 
In this case the amount of the light of the amyl lamp absorbed in the plaster of Paris 
surface was neglected. 
Dr. E. SumpnerI has shown that a jiiece of blotting-paper reflected over 80 per 
cent, of the light incident upon it, and it was found experimentally that the plaster 
of Paris surface of the screen was a still better reflector. 
The current in the discharge vessel was determined, during the measurements of 
the energy, in absolute measure, and also during the comparison of the screen with 
the amyl lamp, in order to correct for changes of intensity of the rays during the 
observations. In this way it was found, using a })latino-barium cyanide screen, 
that 
Intensity of light from 11 norescent screen -QOQg 
Intensity of light froin amyl lamp 
Now if the intensity, I, of the visible light from a Hefner amyl lamp is given by 
I K/r‘' 
from the experiment of Tumlirz§ the value of K for the visible light is equal to 
■00361 gramme calorie per second, 
and the total energy radiated by the lamp is 41'1 times the energy of the light 
radiation alone. 
Now in the exjieriments with the bolometer the heating effect of the rays incident 
‘ Roy. Soc. Edinburgh Proc.,’ 1898. f ‘ Wied. Annal.,’ vol. 37, p. 233. 
1 ‘ Phil. Mag.,’ February, 1893. § ‘ Wied. Annal.,’ vol. 38, p. 640. 
