Published in 1934, this monograph was one of the first introductory accounts of the principles which form the physical basis of the Quantum Theory, considered as a branch of mathematics. The exposition is restricted to a discussion of general principles and does not attempt detailed application to the wide domain of atomic physics, although a number of special problems are considered in elucidation of the principles. The necessary fundamental mathematical methods – the theory of linear operators and of matrics – are developed in the first chapter so this could introduce anyone to the new theory. This is an interesting snapshot of scientific history.