Wave- Length of Hard Gamma Mays. 773 



arbitrary arrangements of the electrons in the atoms of the 

 different elements *, are shown in fig. 1. The measurements 

 for the different wave-lengths were all made at an angle 

 0=90°. 



A in A.U. -* 



Let us suppose that according to these experiments the 

 ratio of the value of yfr for lead to its value for copper is \\ 

 when sin (6/2) /\ = c. Then, if for some unknown wave- 

 length V the value of this ratio becomes R at an angle 0', it 



* Cf. A. H. Compton, Phil. Mag. (soon to be published). 



