270 New Secondary Radiation of Positive Rays. 



If we make R=» + b, write t for t — , and make b inde- 



' a 



finitely great, we get a plane wave-front, the distance of 

 the point B from the plane being p. The right-hand side 

 becomes 



M((ri a . 2-rra/ r\ 2ir . 2W, r\~\ ,_, 



_M pl-cos 

 ~ 2j p I r 



. Z-rraf p 

 XV a 



XXIV. 0?i a i\W Secondary Radiation of Positive Rays. 



To the Editors of the Philosophical Magazine. 

 Gentlemen, 



IN a recent publication (Phil. Mag. (6) xxxv. p. 59, 1918) 

 on this subject I have expressed the belief that the 

 penetrating radiation then observed was the characteristic 

 radiation of tin and lead, and this conclusion was based on 

 the marked differences observed in the intensity of the photo- 

 graphs according to different positions of the foils (see 

 figure /. c). 



Further experiments have not, however, confirmed this 

 supposition, but have led to the discovery of a source of 

 error in my previous researches. When this source of error 

 was eliminated, a uniform slight imprint only could be 

 observed on the photograph. 



Careful investigation of the nature of this radiation was 

 carried out, and the effect of magnetic deflexion on the 

 positive rays shows that the new secondary radiation is 

 excited by the positive ions. From an approximate valuation 

 the coefficient of absorption of the new radiation is estimated 

 to be of the order of that of the characteristic K-radiation of 

 aluminium. 



Yours very truly, 

 The Physical Laboratory, M. WOLFKE. 



Technical High School of Zurich. 

 February 1918. 



