

RADIATION OF ENERGY. 355 



On the other hand, Michelson and Morley 1 using the most refined methods 

 of experimentation were unable to detect any relative motion between the earth 

 and the ether. Their method was, using the interferometer, to send a rav of 

 light a fixed distance, first in the direction of the orbital motion of the earth 

 and then in the opposite direction and try to find a difference in time for the two 

 paths. If the ether exists and is stationary and the velocity of light is inde- 

 pendent of the velocity of the source, it is evident that such a difference of time 

 would exist. This difference of time would produce a shifting of the fringes in 

 the interferometer. But no such shifting could be found. Many other experi- 

 ments both optical and electrical, 2 among which may be mentioned th of 

 Mascart, 3 Rayleigh, 4 Brace, 6 Lodge, 6 and Trouton and Noble, 7 have failed 

 to show any relative motion. 



The assumption of the existence of an ether therefore puts us on the horns of 

 a dilemma. The phenomenon of aberration indicates that the ether is stationary 

 and does not move with the earth, necessitating an ether drift, but when we make 

 experiments to detect this drift we are unable to obtain any evidences of it al- 

 though there are many possible experimental ways in which it could be detected 

 if it existed. 



To explain the failure of the Michelson and Morley experiment to detect this 

 drift, FitzGerald 8 and Lorentz 9 suggested, in 1892, that a body might be shortened 

 in the direction in which it moved. 



A theory which will account for the experimental facts was proposed by 

 Einstein 10 in 1905 and has been called the principle of relativity. 



The principle of relativity rests upon two postulates; the first being that 



all motion is relative, man having no means of detecting absolute motion or 



absolute rest in space; and the second that the velocity of light is independent 



of the motion of the emitting or receiving bodies, and is a universal constant. 



The first postulate, so far as the motion of material bodies is concerned, has been 



recognized as true since Newton's time, and is now extended by the Principle 



of Relativity so as also to cover the motion of material bodies with respect to 



space : this extension being the result of the many efforts which have been made 



in recent years to detect relative motion between the earth and the hypothetical 

 ether. 



These two postulates when subjected to analysis show that, if we assume 



1 Am. Jour. Set., 34, p. 333, 1887. 



• Whittaker, History of the Theories of .Ether and Electricity, 1910, pp. 411-448. 



• Annate de VEc. Norm., (2), 1, 1872, p. 157. 

 4 Phil. Mag., 4, p. 215, 1902. 



• Phil. Mag., Sept., 1905. 



• Phil. Trans., 184, p. 727, 1893; 189, p. 149, 1897. 

 T Phil. Trans., 202, p. 165, 1903. 



• See Lodge, Nature, 46, p. 165, 1892. 



• Verslagen d. Kon. Ak. van Wetenschappen, 3, p. 74, 1892. 

 1 Annalen der Physik, Bd. 17, pp. 891-921, 1905. See also: Jahrbuch der RadioaktiviUt, 4, 411-462, 



1907 



