266 Sir E. Rutherford and Dr. E. N. da C. Andrade: 



constituents o£ radium C*, but does not stop half of the very 

 penetrating rays from radium 0. With the lead plate the- 

 strong line A disappeared, but the group of lines reflected 

 in the neighbourhood of B still remained. We may conse- 

 quently conclude that the radiations with reflexion angles 

 greater than 1° 24' belong mainly to radium B, and the 

 smaller reflexion angles mainly to the penetrating rays from 

 radium C. 



Transmission method of determining wave-lengths. 



While the reflexion method described above left no doubt 

 as to the approximate wave-lengths of the penetrating radia- 

 tions, the uncertainty in regard to the plane or point of 

 reflexion of each type of radiation of the crystal might lead 

 to a possible error of some minutes in the angle of reflexion 

 of a line. This is a considerable percentage error for ai* 

 angle of reflexion of 1° or 2°. 



A number of experiments were made to test whether a 

 more reliable method could be devised for determining the 

 wave-lengths of these very penetrating rays. 



For this purpose, the a-ray tube R was placed behind a 

 rocksalt crystal C, and a photographic plate PP placed at D, 

 the whole apparatus being placed between the poles of the 

 electromagnet to get rid of the effect of /3 rays. 



Fig. 1. 



Suppose, for simplicity, that the source R emits a radiation 

 of one definite frequency which would be reflected at an 

 angle 6 with the surface (cleavage plane) of the crystal. If 

 RE> is the normal to one face (and hence parallel to another 



* See Rutherford and Richardson, Phil. Mag. May 1913, p. 722. 



