I N an article on spinning electrons (NATURE, Jan. 15, p. 90) it was stated that recent progress in atomic physics is mainly due to two primary ideas: the use of a better model of the electron and of a better formal mechanics more suited than classical mechanics to the description of atomic phenomena. The changes and successes consequent on the adoption of this new model of the electron have already been described. We shall try here to give some sort of description of the ideas of the new mechanics and some of the successes to which they have led. We shall try at the same time to exhibit the two independent lines of thought which have led separately to the same new system of mechanics, but by such widely divergent paths that they have justifiably received the different names of matrix mechanics and wave mechanics. The equivalence of these two systems is perhaps the most striking and satisfactory feature of the present development.