Distribution of Secondary Rontgen Radiation. 765 



An examination of the exponential factor o£ (190) is 

 instructive. In the shadow, 6— cos -1 ajr must be positive. 



attaining its minimum when r cos# = a> that is to say, on the 

 line already shown to be the centre of the transitional region t 

 in which diffraction-bands have been indicated. 



At a remote distance the amplitude is proportional to 



*/cos^£-W^ ; 



where 77 is the angle in the figure. 



The evanescence of the exponent in the transitional region 

 indicates the change in character of the solution, which must, 

 for that region, be found by the method previously nsed for 

 the purpose since the method of residues fails. 



We see that for large orientations, the use of raised 

 receivers of the waves does not assist diffraction, to a 

 measurable extent, in giving an effect capable of detection, 

 and our earlier conclusion, that the explanation of long- 

 distance telegraphy must be sought elsewhere, is rendered 

 more emphatic. 



LXXVIII. The Distribution and Quality of the Secondary 

 Rontgen Radiation from Carbon. By H. Pealing, M.Sc, 

 the University of Liverpool * . 



I. Preliminary. 



IF a beam of Rontgen rays of ordinary penetrating power 

 fall on a substance of low atomic weight such as carbon, 

 then this becomes a source of Kontgen rays, which are 

 emitted in all directions and are of the same penetrability as 

 the primary beam. Such a radiation is said to be scattered. 

 * Communicated bv Prof. L. R. Wilberforce. 



