36 
PROFESSOR E. RUTHERFORD AND iMR. R. K. McCLUNG 
on the grid, area 92’2 sq. centims., at a distance of 26 centims. from the source ot 
rays, for the same strength of rays as those incident on the fluorescent screen, was 
•00032 gramme calorie 2 )er second. 
I = K/r^, and the value of K, which represents the amount of energy due to the 
rays falling normally on a surface of 1 sq. centim. at a distance of 1 centim. from 
the source of rays 
= '0023 gramme calorie. 
The intensity of the X rays was thus ‘64 of the intensity of the visible light of the 
standard Hefner lam]). 
Now the efficiency of transformation of X rays into light 
_ energy radiated as light _ ‘0206 x ’OOSGl _ -Qq^ 
energy supplied by the rays '73 x -0023 ’ 
since it was found electrically that '73 of the rays were absorbed in the screen. 
The efficiency of transformation is thus 
4'4 per cent. 
If we assume that 8 5 per cent, of the incident light is diffused from the surface of 
the Lummer-Brodhun screen, the efficiency of transformation is about 3'7 per cent. 
A calcium tungstate screen, in which the absoiq3tion was ‘36, gave almost the same 
efficiencv of transformation. 
The results we have obtained afford a simple means of expressing the intensity of 
X rays in absolute measure, assuming the coefficient of transformation of a fluorescent 
screen to be about 4 per cent. 
Two experiments would l)e necessary— 
(I.) The intensity of the light from the screen would be conqxrred with a Hefner 
standard lamp. 
(2.) The a1)sorption of the rays by the screen would be measured electrically 
or photometrically by placing a 23ortion of the screen to absoi'b the rays. 
Let and L be the intensities of X rays and a Hefner standard lamp in absolute 
measure, disregarding absorj)tion of rays in glass, metal screens, air, &c. When there 
is ecpiality of illumination let and be the distances of the source of rays and 
lamp from the Lummer-Brodhun screen. 
Let 7q and be the distances for equal illumination when the rays pass through a 
])iece of the screen before falling on the Lummer-Brodhun screen. 
Then p, the lutio of transmitted to incident rays for the fluorescent screen, is 
given by 
p = 
Batlo of incident energy absorbed = 1 — p. 
