376 A CENTURY OF SCIENCE 



be overcome is a little greater than when the force is 

 applied at right angles to this direction. Thus we 

 have to distinguish between longitudinal and transverse 

 masses. But the masses of Lorentz's electron are not 

 the same functions of its velocity as those of Abraham's. 

 Kaufmann and after him Bucherer tested experimentally 

 the relation between transverse mass and velocity by 

 observing the deflections produced by electric and mag- 

 netic fields in the paths of high speed beta particles. 

 The latter 's work was such an ample confirmation of 

 Lorentz's formula that it may be considered as proven 

 that a moving electron at least suffers contraction in the 

 direction of motion in the ratio 



Vl - /8 : 1. 



The electromagnetic theory of light had proved so 

 successful when applied to bodies at rest that Lorentz 

 was anxious to extend this theory to the optics of moving 

 media. His problem was to find a group of homogeneous 

 linear transformations that would leave the form of the 

 electrodynamic equations unchanged. The Michelson- 

 Morley experiment had shown that dimensions in the 

 direction of motion must be contracted in the moving 

 system, those at right angles remaining unaltered. But 

 Lorentz soon found that it was also necessary to use a 

 new unit of time in the moving system, and as this time 

 was found to depend upon the position of the point at 

 which it is to be determined, he called it the local time. 

 Lorentz's transformation is just that of the principle 

 of relativity, but he did not succeed in expressing the 

 electrodynamic equations in terms of the new coordinates 

 and time in exactly the same form as for a system at 

 rest, for the reason that he failed to endow these new 

 units with sufficient reality to justify him in using them 

 when it came to transforming the velocity term involved 

 in an electric current. 



Principle of Relativity. In 1905 appeared in the 

 Annalen der Physik 14 a paper destined to alter entirely 

 the point of view from which problems in light and elec- 

 tromagnetic theory are to be approached. The author 

 was Albert Einstein, of Berne, Switzerland, a young man 



