Stopping Power and Atomic Number. 477 



dividing by the focal length of the camera, the values o£ 6 P 

 and therefrom the mean i, i p , and cos i p for each value of p 

 are evaluated. In each case 8i p is given by the measured 

 displacement on the plate of the metallic fringe from the 

 normal system divided by the focal length of the camera 

 lens. The mean of the values of cos i p 8i p for each of the 

 values of p is the value for cos i Si employed in the final 

 substitution in (9) to give the value of the phase difference. 



In practice, the method works out quite satisfactorily, and 

 the preliminary study in the case of silver and mercury has 

 yielded quite concordant results. Because of freedom from 

 surface impurities in the present method, and considering 

 that the effect of surface impurities is to lower the values, 

 the slightly higher values obtained by this method are easily 

 explained. This view has also in corroboration the fact of the 

 observed difference in the case of mercury being much smaller 

 than with silver, which is more liable to surface contamination 

 than mercury. Further, the method has also the advantage 

 of being applicable to all the other metals which can be 

 coated on to the Interferometer plate by cathodic deposition 

 or otherwise. A detailed evaluation of this phase difference 

 for all the metals is in progress. 



In conclusion, I beg to express my indebtedness to Prof. 

 R. LI. Jones, M. A., Professor of Physics, Presidency College, 

 Madras, for his kind and sympathetic help during the progress 

 this investigation at Madras under his direction. 



University College, 

 London. 



LIII. Stopping Power and Atomic Number. 

 By J. L/Glasson, M.A., D.Sc* 



1. TT has recently been established that certain properties 

 _L of the atom are more simply and accurately expressed 

 in terms of atomic number than in terms of atomic weight. 

 The following is an attempt to find our, if this is also true of 

 the atomic property known as stopping power for u rays. 



According to the work of Sir William Bragg, it was shown 

 that the stopping power of an atom is proportional to the 

 square root of the weight oi the atom. Professor Bragg's 

 own figures which form the basis of the work are given in, 



* Communicated by Prof. Sir E. Rutherford, F.R.S. 



