1. Philosophy of Science, 27, 233 (1960).
2. Samuel, Viscount, and Dingle, H., A Threefold Cord, 270 (Allen and Unwin, 1961).
3. Ann. Phys., 17, 891 (1905): the translation given here is taken from The Principle of Relativity, by A. Einstein et al., 49 (Methuen, 1923).