712 PKOCEEDINGS OF THE AMERICAN ACADEMY. 



This possibility being excluded, the only satisfactory explanation of 

 the Michelson-Morley experiment which has been offered is due to 

 Lorentz, 3 who assumed that all bodies in motion are shortened in the 

 line of their motion by an amount which is a simple function of the 

 velocity. This shortening would produce a compensation j ust sufficient 

 to offset the predicted positive effect in the Michelson-Morley experi- 

 ment, and would also account for the result obtained by Trouton and 

 Noble. It would not, however, prevent the determination of absolute 

 motion by other analogous experiments which have not yet been tried. 



Einstein 4 has gone one step farther. Because of the experiments 

 that we have cited, and because of the failure of every other attempt 

 that has ever been made to determine absolute velocity through space, 

 he concludes that further similar attempts will also fail. In fact he 

 states as a law of nature that absolute uniform translatory motion can 

 be neither measured nor detected. 



The second fundamental generalization made by Einstein he calls 

 " the law of the constancy of light velocity." It states that the 

 velocity of light in free space appears the same to all observers, regard- 

 less of the motion of the source of light or of the observer. 



These two laws taken together constitute the principle of relativity. 

 They generalize a number of experimental facts and are inconsistent 

 with none. In so far as these generalizations go beyond existing facts 

 they require further verification. To such verification, however, we 

 may look forward with reasonable confidence, for Einstein has deduced 

 from the principle of relativity, together with the electromagnetic theory, 

 a number of striking consequences which are remarkably self-consistent. 

 Moreover the system of mechanics which he obtains is identical with 

 the non-Newtonian Mechanics developed from entirely different prem- 

 ises by one of the present authors. 5 Finally, one of the most important 

 equations of this non -Newtonian mechanics has within the past year 

 been quantitatively verified by the experiments of Bucherer 6 on the 

 mass of a /5 particle, to which we shall refer later. 



Therefore, in as far as present knowledge goes, we may consider the 

 principle of relativity established on a pretty firm basis of experimental 

 fact. Therefore, accepting this principle, we shall accept the conse- 



3 Abhandlungen fiber Theoretische Physik, Leipzig, 1907, 443. 



4 An excellent summary of the conclusions drawn from the principle of 

 relativity, by Einstein, Planck, and others, is given by Einstein in the Jahr- 

 buch der Radioaktivitat, 4, 411 (1907). An interesting treatment of certain 

 phases of this problem is given by Bumstead, Amer. Jour. Sci., 26, 493 (190S). 



5 Lewis, Phil. Mag., 16, 705 (190S). 



6 Ber. Phys. Ges., 6, 688 (1908); Ann. Physik, 28, 513 (1909). 



