This paper contributes to historical bibliometrics in proposing to apply social networks methodology and the concept of centrality to co-citation networks. Using the case of physics for the period 1900-1944 we show that measuring the centrality of authors in co-citation networks provides a useful index of the evolution of a scientific field (in this case physics) and the changing focus of research over time. We divided the 45-year period into seven sub-periods related to major events in physics and calculated the centrality of actors present in the co-citation network for each of these periods. The results obtained reflect the major changes in the disciplines: we see the evolving rise and decline in centrality of major physicists like H.A. Lorentz, A. Einstein, N. Bohr, E. Ruherford as well as others less known figures as the « hot » topics move from black body and electron theory to relativity and spectroscopy and from quantum mechanics to nuclear physics.