Prof. E. Rutherford and Mr. I, 1 . K. M clung. 



or the rate of emission of energy per second as visible light from the 

 Hefner lamp is nearly twice the rate of emission of energy from the 

 X-ray tube. 0'73 of the energy of the rays was absorbed in the 

 screcMi. 



The efficiency of the transformation of X rays into visible light by 

 the screen (compared with the Hefner lamp) 



= 0*044 or 4-4 per cent. 



Assuming this transformation factor for a fluorescent screen, two 

 simple photometric measurements are required to express the energy of 

 any bulb in absolute measure. The light from a fluorescent screen is 

 first compared with the standard Hefner lamp. The absorption of the 

 rays in the screen is determined by placing a piece of the screen in the 

 path of the rays. 



Let p = ratio of intensities of light from bulb and lamp, 



pi = ratio of transmitted to incident radiation on the screen. 



Then it is shown that the intensity in absolute measure 



0-082p , . 



= - E gramme-calorie per second. 



1 -pi 



The absorption in the cardboard of the screen is supposed to be 

 negligible, but if necessary can be readily allowed for. 



Absorption of X Rays in Gases. 



A null method was employed, as the absorption of the rajs in air at 

 atmospheric pressure was small. The rays passed through two long 

 brass tubes with aluminium ends, and the current produced by the 

 rays, after passing through one tube, was balanced against the current 

 due to the other. On exhausting one tube the electrometer balance 

 was disturbed. From measurements of the deflection per second from 

 the balance and the deflection per second due to the rays after passing 

 through one tube, the absorption can be calculated. The mean value 

 of the coefficient of absorption of the rays in air at atmospheric 

 pressure was found to be 



0-000279, 



or the rays would pass through 24'7 metres before absorption reduced 

 the intensity of the radiation to one-half. 



The absorption was found to be proportional to the pressure from a 

 hulf atmosphere to three atmospheres. 



The coefficient of absorption in carbonic acid gas was found to be 

 1'59 times the absorption in air. 



