Manchester lilginoirs, Vol. liv. (1910), No. 14. 9 



A. Einstein* has developed this theory of relativity 

 starting with the fundamental idea of the constancy of 

 the velocity of light, and has thus been able to present us 

 with a new kinematics which is apparently more con- 

 sistent with the modern theories of electrodynamics than 

 the approximate kinematics to which we are accustomed. 

 Some of the most interesting results of the theory are that 

 the resultant of two velocities, both of which are less than 

 that of light, is always a velocity less than that of light ; 

 the resultant of two velocities one of which is equal to 

 that of light is a velocity equal to that of light ; the resultant 

 of two velocities equal to that of light but of opposite 

 directions is indeterminate, and may have any value less 

 than or equal to that of light. 



This theory of relativity is not based simply on 

 theoretical considerations ; it has received considerable 

 support from some very delicate experiments. It was 

 first put forward in an approximate form by Lorentzf 

 and Larmorj, following a suggestion made by FitzGerald§, 

 to explain the negative results of the Michelson-Morley 

 experiment. It was then found that it provided an 

 ample explanation of a number of other negative results 

 concerning the effect of the motion of the Earth on 

 double refraction li, the rotation of the plane of polarisa- 

 tionlT, the resistance of a piece of metal**, and other 

 physical phenomena. Also, the theoretical formula 



*Ann. der. Physik, vol. 17 (1905). Jahrb. der Radioaktivildt (1907). 



t " Versuch einer Theorie der elektrischen und optischen Erscheinungen 

 in bewegten Korpern." Leiden. (1895.) 



t Aether and Matter. (1900.) Ch. x., xi., xiii. 



§ Public lectures in Trinity College, Dublin. 



II D. C. Brace Phil. Mag., (6), vol. 7, p. 317, 1904. 



ITRayleigh. Phil. Mag., (6), vol. 4, p. 215. Brace. Phil. Mag., (6), 

 vol. 10, p. 383, 1905. Ibid., p. 391. 



** Trouton and Rankine. Phil. Tians. A. (1908), p. 420. 



