The Common Sense of Relativity. 503 



with experiment might vet (as T hold) be judged objectionable 

 on other grounds, and, conversely, a theory which was not in 

 complete accord with experiment might yet be judged 

 satisfactory. 



The special laws which it is the business of the Principle 

 of Relativity to explain (that is, those which it is specially 

 important to be able to deduce from the theory) are those 

 which are met with in the study of the optical and electrical * 

 properties of systems in relative motion, but in this case, as in 

 most cases, it turns out that laws other than those con- 

 templated originally are deducible from the theory. It is 

 important to notice that there is another theory, that of 

 Lorentz f, which explains completely all the electrical laws 

 of relatively moving systems, that the deductions from the 

 Principle of Relativity are identical with those from the 

 Lorentzinn theory, and that both sets of deductions agree 

 completely with all experiments that have been performed J. 

 If, then, anyone prefers one theory to the other it must be 

 either on the ground of differences in the laws not con- 

 templated originally which are predicted respectively by the 

 two theories, or because of some general grounds independent 

 of experimental considerations. 



3. The fundamental propositions of which the theory 

 consists will now be enumerated and a few remarks made 

 upon each. For the sake of brevity, I shall call a system the 

 parts of which are all relatively at rest a a quiet"" system, 

 and one of which the parts are in relative motion a " disturbed " 

 system. The terms are convenient to distinguish quiet and 

 disturbed systems from those which are moving as wholes 

 relatively to each other. Two quiet systems may be in 

 relative motion as wholes. 



(A.) The first assertion of the Principle of Relativity 

 concerns quiet systems only. It asserts that any law and, 

 consequently, any theory from which laws can be deduced, 

 which has been found to hold for one quiet system which 

 includes all the particles of which mention is made in the 



* " Optical " and '' electrical " will be employed throughout as 

 equivalent terms, one or the other being used according to the context. 



f As given in the Enci/clo. d. Mathe.mat . Wissen. 



\ This statement is only true if quantities of a higher order than the 

 second in v/c are left out of account. Since such terms cannot be detected 

 experimentally the conclusion given is not affected, and the terms will be 

 neglected throughout our argument. In saying that both theories agree 

 completely with experiment, I do not wish to offer any opinion as to the 

 value of the experiments of Bucherer and others which have been 

 subjected to critisism : I merely assume that the results announced in 

 them will be accepted ultimately, because, if they are not. the Principle 

 of Relativity would seem to be unworthy of further discussion. 



