BOOK REVIEW 81 account of creation, acceptance and resistance on the part of the giants of mathematical physics of that age is written without any undue adulation and seenrs to be backed by facts. As regards the controversy concerning priority in the special theory which Whittaker attributes to Lorentz and Poincare, Pais correctly dismisses this with the apt sentence that the "author's lack of physical insight matches his ignorance of the literature'5 (p 168). One must add however that Einstein did not view the development of the theory of relativity as a personal triumph. He characteristically viewed its development in a historical perspective, asserting that "it seems obvious that by 1905 it was ripe for discovery". Pais discusses Einstein's epoch-making discovery in 1905 of the photo-electric effect according to which light is not merely emitted or absorbed discontinuously (which was shown by Planck in 1900) but actually consists of discrete, indivisible portions—light quanta known as photons. His later attitude to the development of the quantum theory, for which he provided the foundation stone, was summed up by him in 1951, "All these fifty years of pondering have not brought me any closer to answering the question, what are light quanta?55 His third major achievement of 1905 was the study of Brownian motion which secured general acceptance of the reality of molecules. The significance of this work Einstein located in the fact "that one sees directly under the microscope part of the heat energy in the form of mechanical energy55 and that "all doubts vanished about the correctness of Boltzman's interpretation of the thermodynamic laws". What is remarkable about Einstein's creativity of this period is the ease and elan with which he introduced new concepts which Lorentz, Poincare, Macli and Ostawald, to name a few, found difficult to accept. For instance, the reality of the atomistic constitution of matter was conceded by Ostawald in 1908 and Mach was never convinced of it. Einstein's work from 1908 to 1915 was mainly devoted to the working out of the general theory of relativity which incorporated the generalisation from invariance under uniform motion and invariance under general motion. The scientific task that Einstein set himself in his later years was based on three desiderata, all of them vitally important to him: to unify gravitation and electro-magnetism, to derive quantum physics from an underlying causal theory, and to describe particles as singularity-free solutions of continuous fields. The task of unification of forces that he set himself and which he pursued tenaciously with an unparalleled intellectual conviction^ considering the absence of success in his own life time, is now widely recognised to be one of the important tasks in physics, perhaps the most important one. Electromagnetism has been joined not to gravitation but to weak interactions and the unification of gravitation to other