CONTEMPORARY ADVANCES IN PHYSICS 87 



upon lithium or paraffin give the best "yield"; visible light, none 

 perceptible. 



The shifted X-rays scattered at 90° are polarized, very nearly com- 

 pletely (Lukirsky, Kallmann and Mark). 



The Scattering of Quanta Attended by Extraction of 

 Bound Electrons 



Almost immediately after Arthur Compton had measured and in- 

 terpreted the scattering which is due to collisions of quanta with free 

 electrons, he and others realized that corpuscles of light might possibly 

 encounter atoms in such a way that they extracted bound electrons, 

 and thereupon were scattered with a corresponding abatement of their 

 energy. In fact it seemed most probable that the electrons responsible 

 for the Compton effect were themselves not quite free, but very lightly 

 bound; and that a careful study of the scattered quanta, the "shifted" 

 or "modified" line, would reveal that they had spent energy in dis- 

 solving the bonds as well as in imparting kinetic energy to the electrons. 

 Researches on this topic were numerous, and are still continuing. At 

 that time, the Raman effect had not yet been discovered; and perhaps 

 it did not seem natural to accept the transfer of energy from cor- 

 puscles of light to atoms as a general phenomenon, apart from special 

 cases so easy and so beautiful to visualize as the elastic impacts of 

 quanta against free electrons. At all events, when in 1923 and 1924 

 data were published by G. L. Clark and W. Duane which to the 

 present-day onlooker seem to declare the effect in the most forthright 

 fashion, they made no such impression. 



The experiments of Clark and Duane were involved in a long contro- 

 versy, in which the reality even of the Compton effect was called into 

 question. Data were obtained by some experimenters, which others 

 could not or at least did not reproduce. The questions at issue 

 speedily reached the point, where no outsider could risk a judgment 

 unless he was himself a great expert in the study of X-ray scattering. 

 Unfortunately the experiments were terminated, when the reality of 

 the Compton effect was established. I say "unfortunately," for it now 

 seems as if in the nature of things both sides must have been right. 

 Compton had discovered the transfer of energy from quanta to free or 

 nearly free electrons; Clark and Duane must have discovered the 

 transfer of energy from quanta to bound electrons, or the process in 

 which a corpuscle of light uses part of its energy in ionizing an atom, 

 part in giving speed to the liberated electron, and retains the remainder. 

 It would be a very desirable result of the present-day revival of interest 

 in scattering, if somebody should reinvestigate this entire field. 



