Henry Andreivs Bumstead. 473 



of gravitational force should produce a first order pertur- 

 bation in planetary orbits. 



While Bumstead was greatly impressed by the beauty 

 and symmetry of Einstein's theory, the ether had such a 

 real signiticance to him that he was never able to accept 

 completely the view-point of the relativist. Furthermore, 

 he doubted the value of the new principle in opening up 

 unexplored fields of research. To him it seemed like a 

 closed system, perfect but infertile. Hence Einstein's 

 ultimate success in generalizing the principle, so as to 

 make possible the application of the equivalence hypoth- 

 esis to gravitational fields, appealed to him all the more 

 as a great work of genius. 



In 1911 Bumstead turned his attention to a study of the 

 delta rays emitted by metals under the influence of alpha 

 rays, which he continued for the three following years. 

 Delta rays — so named by J. J. Thomson — are the slow- 

 moving electrons detached from metallic atoms by the 

 impact of the more massive alpha particles. The ioniza- 

 tion curves obtained by Bumstead show all the character- 

 istics of the Bragg curves for gases, but unlike the latter, 

 the curves for different metals have very closely the same 

 form. This led him to suspect that the delta rays come 

 from a gas absorbed on the metal surface. An investiga- 

 tion of the velocities of the particles constituting the 

 rays revealed the fact that some of them have velocities 

 corresponding to a potential difference as great as 2000 

 volts. These swifter rays seem to be the primary result 

 of impact of alpha rays, and to give rise to secondary 

 slow-moving electrons when they collide with other atoms. 

 The result of this experiment suggested to him that fast- 

 moving electrons may also be produced when gaseous 

 molecules are struck by alpha particles. To investigate 

 this matter, he obtained from England an expansion 

 apparatus made after C. T. E. Wilson's design. This 

 apparatus he modified so as to enable him to work in 

 hydrogen at a pressure of 100 mm., and with it he obtained 

 a number of photographs of alpha ray tracks, w^hich 

 showed very clearly electronic trails radiating from the 

 column. These trails are undoubtedly due to swift delta 

 rays. 



In addition to the papers published under his own name, 

 Professor Bumstead supplied the underlying ideas and 

 much of the motive force responsible for the great 



