37 
ON ENERGY OF RONTGEN AND BECQ.UEREL RAYS, ETC. 
Therefore it is readily seen that 
h 
L 
X 
100 
- p) 4-4 
since efficiency is 4'4 per cent. 
Thus, since I 3 = ‘00361 gramme calorie, 
2 
Ii = ‘082 - -r oTamme calorie. 
1 - p)^ 
Thus two simple j^hotometric com|)arisons would be required to express the energy 
of the radiation of any particular bulb in absolute measure. 
For penetrating rays the absorption in the cardboard of the screen is negligilde, ljut 
if necessary it can readily be allowed for. 
The chief source of difficulty in the comparison is the difterence in colour betw'eeu 
the light from the Hefner lamp and a fluorescent screen. The fluorescent light 
appears a greenish-blue, and the amyl lamp a reddish-j^ellow, when seen side by side 
in the telescope of a Lummer-Brodhun screen. 
Some experiments were made using a coloured glass to make the sources of liglit 
more nearly a match in colour. A greenish coloured glass was found to give a good 
colour match when interposed between the screen and amyl lamp. On determining 
by means of a thermopile the amount of the visible energy of the Hefner lamp which 
was allowed to go through the glass, it was found to be so small (less than 2 per 
^ent.) that a special investigation was required to find the coefficient of transmission 
with accuracy. The experiments were not continued, owing to lack of time, but the 
evidence showed that by using a coloured screen in the path of the amyl lamp to give 
the same tint as the fluorescent screen, the efficiency of the transformation of the 
screen wais much higher in that case than the 4 per cent, obtained by using no 
coloured glass or solution. 
Energy required to lyroduce an Ion. 
The method employed to determine the energy required to produce an ion in gases 
exposed to Bhntgen rays depended on the measurement of the heating effect of the 
rays, and of the total number of ions produced l^y the radiation in the gas. If H is 
the number of calories given out per second by the rays, and E the energy in ergs 
of the rays emitted per second, then 
E = JH.(1). 
If W is the average amount of energy required to produce an ion, then 
nW = E.(2), 
where n is the number of ions produced per second, suj^posing that all the energy of 
the rays, absorbed in the gas, is due alone to the production of ions. 
