766 Mr. H. Pealing on Distribution and Quality of 



The intensity of the secondary rays from the carbon varies 

 in different directions. In directions nearly parallel to the 

 incident primary beam the intensity is greatest, while in a 

 direction at right angles it is least. 



The objects of this investigation were : — 



(1) To find the variation in the distribution of the rays 



emitted in the horizontal plane containing the primary 

 rays, when (a) the primary beam was gradually hardened, 

 and when (b) the harder portions of the primary beam 

 were used ; and 



(2) To compare the absorbability of the carbon radiation with 



the primary producing it, and to find how the difference 

 alters when the primary is made very hard and only 

 the hard portions of it are used. 



II. Theory of the Distribution of the Secondary Radiation 

 from Carbon. 



Several investigators, notably Barkla *, have worked out 

 some results of the elementary theory of the distribution of 

 a scattered radiation, on the assumption that the rays consist 

 entirely of electromagnetic impulses travelling with the 

 velocity of light. 



A single aether pulse travelling in a horizontal direction 

 with the direction of its electric force vertical, will cause an 

 atom of carbon to radiate the rays with equal intensity in all 

 directions in the horizontal plane containing the atom ; while 

 the radiation set up in it by a pulse having the direction of 

 electric force horizontal will be zero in a direction at right 

 angles to the beam, and a maximum in the same direction as 

 the primary rays. When the incident beam is on the whole 

 un polarized, the intensity of the scattered radiation in this 

 horizontal plane in a direction making an angle u with the 

 primary beam will be 



I a = I 90 (l + cos 2 a), 



where I a and I 90 are the intensities in directions making 

 angles of ol and 90° respectively with the primary beam. 



This law was tested by Barkla and Ayresf , and found by 

 them to be accurately obeyed for all values of a greater 

 than 30° when the primary beam is very soft. With angles 

 less than 30° the ratio was considerably greater than the value 

 predicted by theory. W T hen the primary beam is harder, 



* Barkla, Phil. Mag. Feb. 1901, pp. 288-296. Barkla and Ayres, Phil. 

 Mag. Feb. 1911, pp. 270-278. 

 f Loc. tit. 



