332 ILLINOIS STATE ACADEMY OP SCIENCE 



INTRODUCTION 



Some years ago J. J. Thomson advanced a theory of 

 light which had properties characteristic of both the emis- 

 sion theory and the usual form of the undulatory theory. 

 While lecturing in 1911, he proposed as an experimental 

 test to the theory that if a stream of electrons had a 

 strong beam of light thrown directly across their path 

 slight deflections of the electrons might be expected. C. 

 T. Knipp/ who w^as a student with Thomson at the time, 

 saw the possibilities of such a research and soon after his 

 return to Illinois attempted the experiment. "He used a 

 cathode beam twisted into a spiral, by means of a mag- 

 netic field, which fell on a photographic plate leaving a 

 trace in the form of a circle. Although much work was 

 done at that time by Knipp and later by 0. A. Randolph 

 (1912), and also by C. F. Hill (1915), yet the difficulties of 

 obtaining high vacua, together with the great mechanical 

 complications, prevented conclusive results from being 

 obtained. 



Owing to the fact that since that time some prominent 

 physicists^ have modified their views concerning the 

 electromagnetic theory of light^ and much has been Avrit- 

 ten concerning non-continuous electromagnetic wave 

 fronts, it was thought that it might be profitable with the 

 modern methods of producing high vacua to carry for- 

 ward the experiment. To that end the old apparatus was 

 redesigned and entirely rebuilt in an attempt to deter- 

 mine whether or not electromagnetic radiation has an 

 effect on rapidly moving electrons when thrown across 

 their path. 



DESCEIPTION OF APPAEATUS 



The general arrangement of the apparatus is shown 

 in Fig. 1. The electron discharge chamber Avas con- 

 structed from a cylindrical glass jar 9.2 cm. in diameter 

 and 76 cm. long, inside measurements. A powerful mag- 



1. C. T. Knipp, Phys. Rev., Vol. 34, p. 477. 



2. H. Bateman, Phil. Mag., Vol. 250, p. 405 (1917) ; A. Einstein, Phys. 

 Zeitschr., V. 18, p. 121, (1917). 



3. J. Kunz, Phys. Rev., 2d Ser., V. 3, p. 464; J. J. Thomson, Froc. Camb. 

 Phil Soc, V. 14, p. 421, Presidential Address, Brit. Assoc, Winnipeg (1909), 

 Phil. Mag., Feb. (1910), Oct. (1913); H. S. Allen, Phil. Mag., Oct. (1921), 

 p. 523. 



