WHATEVER view is held regarding the nature of light, it must. now be admitted that the process whereby an atom loses radiant energy, and another near or distant atom receives the same energy, is characterised by a remarkable abruptness and singleness. We are reminded of the process in which a molecule loses or gains a whole atom or a whole electron but never a fraction of one or the other. When the genius of Planck brought him to the first formulation of the quantum theory, a new kind of atomicity was suggested, and thus Einstein was led to the idea of light quanta which has proved so fertile. Indeed, we now have ample evidence that radiant energy (at least in the case of high frequencies) may be regarded as travelling in discrete units, each of which passes over a definite path in accordance with mechanical laws.