a Plate exposed to a Beam of X-rays. 213 



Besides cf> is in a close connexion with the well-known 

 function 



W Jo lo g* 



which under the denomination of integral logarithm has 

 been the object of special study. Indeed, we get by exchange 

 of variables 



<j)(\)=-eH(e^) (19) 



The expression obtained allows of calculating, without any 

 difficulty, the scattered radiation, which under a given visual 

 angle penetrates into the ionization-chamber, as soon as we 

 know the intensity, absorption and scattering coefficient of 

 the incident radiation. It is evident that we may by the aid 

 of this expression be able to calculate the said coefficients 

 from the experimental determinations. I leave to the expe- 

 rimental physicists the consideration of how this may be beat 

 executed in practice. 



In conclusion I wish to make a numerical application of 

 the above results to some data taken from the experiments of 

 Auren. He placed the plate in two differ ent positions, and 

 measured the radiation entering the ionization-chamber in 

 both cases. The distance to the centre of the plate was in 

 the former position 2 - l cm., in the latter 12*3 cm. The 

 distance between the ionization-chamber and the focus of the 

 bulb was 42 cm. In one case Auren examined a graphite 

 plate 1 cm. thick. The mass-scattering coefficient for C, 

 according to Auren, is 0142. Thus the scattering coefficient 

 a — 1*7 . 1 42, because the density is about 1*7. For one of the 

 wave-lengths used by him (034 . 10" 8 cm.) k =0094 . 1*7. 

 Thus 



/j = /(k + c7) = 0-4. 



By the aid of these facts, I get the following values of K 

 in both positions : 



Ki = 0'0997, 



K 2 =O00408. 

 For the sake of comparison, I calculate K^, and get 

 K =1-328. 



00 



The scattering leaving the plate in the different positions 

 is moreover inversely proportional to the square of the 

 distance of the plate from the focus. With this in view, I 

 calculate the ratio of scattered radiation entering the chamber 



Phil Mag. S. 6. Vol. 37. No. 218. Feb. 1919. Q 



