might be transferred without loss, indirectly from the light-wave to the conduction electron, thus obviating the necessity of a direct transfer. In other words, the Klein and Rosseland discovery proved that the energy
could be transferred from the light-wave to the conduction electron by being absorbed first by an atom, which would thus be changed from the normal to the excited state, i.e., the state in which one of its electrons has been lifted from a normal to an outer orbit. This excited atom could then return to its normal state without radiation by a collision “of the second kind,” which consists in transferring its whole absorbed energy
to a free or conduction electron. The reality of this phenomenon has been experimentally checked by Franck and Cario.[188] This important discovery then left the evidence for localized light-quanta precisely where it was before.[189]
Within the past year, however, a young American physicist, Dr. A. H. Compton, of the University of Chicago, has discovered another new phenomenon which constitutes perhaps the best evidence yet found in favor of Einstein’s hypothesis of localized light-quanta.
Compton’s procedure is as follows. Assuming, for the sake of obtaining quantitative relations, the correctness of Einstein’s hypothesis, he argues that when such a “light-quanta” collides with a free electron the impact should be governed by the laws which hold for the collision between any material bodies. These are two in number, namely: (1) the principle of the conservation of energy; (2) the principle of the conservation of momentum (Newton’s Third Law).
Now the energy of a light-quanta, as heretofore shown, is