Spectrum of the ft Rays excited by <y Rays. 281 



unmistakable bands in a single photograph taken with an 

 aluminium radiator. 



The complete investigation will probably show the different 

 /3-ray spectra to be extremely complex, and the question of 

 their connexion with the corresponding complexity of the 

 X-ray spectra is o£ the greatest theoretical importance, 

 calling for closer examination of both the X ray and the 

 excited /3-ray spectra of the elements. It will be observed 

 that the iron line (1) corresponds to a wave-length only 

 2 per cent, greater than the value 1*946 x 10" 8 cm. given by 

 Moseley * for the a line of iron, and the line (2) of lead to 

 a wave-length greater by 1 per cent, than that of the a line 

 of nickel (1'662 x 10" 8 cm.). These differences are no 

 greater than can be accounted for by uncertainties in tbe 

 numerical constants involved : also, on account of the faint- 

 ness of the photographic impression, it is difficult to measure 

 very accurately the radius of curvature of the path of the 

 excited ft rays, and any percentage error introduced here is 

 doubled in the calculation of the energy. The evidence, 

 therefore, so far as our experiments go, indicates the pro- 

 duction of excited ft rays characteristic both of the anti- 

 cathode and of the radiator. Whether the complete /3-ray 

 spectra can be connected with the X-ray spectra on the 

 simple quantum theory, or whether the latter will have to be 

 modified to bring the spectra into line, is a question to be 

 solved by further experiment. 



In conclusion we have much pleasure in thanking Sir 

 Ernest Rutherford for suggesting this experiment, and for 

 his advice throughout the course of the work. 



Physical Laboratories, 



University of Manchester. 

 July 1st, 1914. 



XXXIV. Spectrum of the ft Rays excited by 7 Rays. By 

 Sir Ernest Rutherford, F.R.8., H. Robinson, M.Sc, 

 and W. F. Rawltnson, B.Sc, University of Manchester f. 



IT is well known that the primary ft radiation emitted by 

 the great majority of radioactive substances contains 

 a number of groups of rays expelled with definite velocities. 

 When the ft rays are analysed by their passage through 

 a magnetic field, and received on a photographic plate, they 



* Moseley, Phil. Mag. xxvi. p. 1024 (1913). 

 t Communicated by the Authors. 



