  In the last section we saw some of the difficulty inherent in experiment-theory comparison. One is sometimes faced with the question of whether the experimental apparatus satisfies the conditions required by theory, or conversely, whether the appropriate theory is being compared to the experimental result. A case in point is the history of experiments on the double-scattering of electrons by heavy nuclei (Mott scattering) during the 1930s and the relation of these results to Dirac’s theory of the electron, an episode in which the question of whether or not the experiment satisfied the conditions of the theoretical calculation was central. Initially, experiments disagreed with Mott’s calculation, casting doubt on the underlying Dirac theory. After more than a decade of work, both experimental and theoretical, it was realized that there was a background effect in the experiments that masked the predicted effect. When the background was eliminated experiment and theory agreed.  (Appendix 6)