292 Mr. Norman Campbell on Delta Bays. 



the copper appears to emit more slow rays, but also more 

 fast rays than the aluminium. If then the curves can be 

 taken as a measure of the speed of the delta rays, the very 

 interesting conclusion appears indicated that the speed of the 

 delta rays depends on the material from which they are 

 emitted, and not on the velocities of the alpha rays exciting 

 them. The delta rays would seem to be a true secondary 

 radiation. 



14. Bat before this conclusion can be accepted, the 

 source of uncertainty must be discussed which has only been 

 indicated hitherto. This source is the reflexion of the 

 rays by the electrode, which was proved to exist for slow 

 moving electrons by v. Baeyer*. Do the curves given indicate 

 anything about the velocity of the delta rays, or do they 

 depend entirely upon varying amounts of reflexion at the 

 electrodes ? 



There can be no doubt whatsoever f rom v. Baeyer's figures 

 that reflexion will alter the form of the curves very materially. 

 On this ground it is not pretended that the curves given can 

 be taken as a measure of the proportion of rays of a given 

 speed. But the measurements have all been performed under 

 the same conditions, and conclusions have only been drawn 

 from comparisons of different curves, not on comparisons of 

 the same curve at different points. Are these comparisons 

 rendered unjustifiable by the existence of reflexion ? 



It is not very easy to predict what would be the result of 

 reflexion. In v. Baeyer's experiments the reflected rays were 

 not subjected to an electric field, so that his results are not 

 immediately applicable. Indeed he obtained intense reflexion 

 when the rays were moving with a velocity corresponding to 

 40 volts, whereas the perfect saturation obtained in these 

 experiments with that potential shows that the effects of re- 

 flexion are rendered negligible by the action of the field in 

 retaining on the electrode any rays which strike it. For 

 lower potentials the reflexion will have two effects. It 

 will tend to decrease the number of rays moving up to an 

 electrode against the electric field and retained by it; for some 

 of them will be reflected and travel away with the field. 

 On the other hand, it may cause some of the rays which have 

 been emitted by the electrode to return to it after reflexion at 

 the other electrode, even though the electric field tends to 

 prevent the return. The first action will make the rays 

 appear more easily stopped by an electric field than they 



* v. Baeyer, Phijs. Zeit. x. p. 176 (1909), and earlier papers. Cf. also 

 Ladenburg and Marliau, Phys. Zeit. ix. p. 821 (1908). 



