﻿804 Mr. G. Shearer on the 



these experiments; and these results are to be taken as repre- 

 senting what happens when the characteristic radiations are 

 not excited to any extent. 



In view of De Broglie's results, it is to be expected that 

 many of the electrons emitted in connexion with the 

 characteristic radiations will have relatively small velocities, 

 and will, therefore, have difficulty in escaping from the 

 radiator. Only a small fraction of those liberated will 

 escape and contribute to the effect measured in these experi- 

 ments. 



(7) Selective Emission in the Direction of Electric Vector. — 

 On the classical electromagnetic theory of light it seems 

 probable that there will be a large preponderance of electrons 

 emitted in the direction of the electric vector in the X-ray 

 beam. In fact, the photographs obtained by C. T. R. Wilson 

 seem to show that such is the case. In order to find evidence 

 for such an effect, two adjacent sides of the cube were covered 

 with the same metal, and measurements of the electron 

 emission were made when the rays fell perpendicularly and 

 at an angle of 45° on the metal. In the second case the area of 

 metal under bombardment was 1*4 times the area in the first 

 case, and for this reason an increase of 40 per cent, in the 

 emission is to be expected. When the rays fall perpen- 

 dicularly, electrons emitted in the direction of the electric 

 vector should have difficulty in escaping from the metal, but 

 when the angle of incidence is 45° this difficulty should not 

 be so marked. 



The results of such measurements made with lead as 

 radiator showed an increase of 35 per cent, when the angle 

 of incidence was 45°. This increase is rather more than 

 accounted for by the increase of area of the radiator, and 

 these observations show no evidence of any selective emission 

 in the direction of the electric vector. It is probable that 

 by the time that the electrons emerge from the metal their 

 direction of motion is very different from what it was initially, 

 owing to encounters with the atoms of the metal ; and experi- 

 ments such as these could not be expected to throw light 

 on the initial direction of motion of the electrons. Wilson's 

 photographs were taken with a gas as source of electrons, and 

 in this case the initial direction of the electron is directly 

 observed. 



(8) Variation of Electron Emission with Applied Voltage. — 

 A few experiments were carried out to see how the number 

 of electrons emitted per unit intensity from any one radiator 



