Closing Years of the Nineteenth Century. 417 



the terrestrial motion to the opposite direction. This was 

 afterwards confirmed by Lord Kayleigh,* who found that the 

 alteration, if it existed, could not amount to (l/100,000)th 

 part. 



In terrestrial methods of determining the velocity of light 

 the ray is made to retrace its path, so that any velocity which 

 the earth might possess with respect to the luminiferous medium 

 would affect the time of the double passage only by an amount 

 proportional to the square of the constant of aberration.f In 

 1881, however, A. A. MichelsonJ remarked that the effect, 

 though of the second order, should be manifested by a measur- 

 able difference between the times for rays describing equal 

 paths parallel and perpendicular respectively to the direction of 

 the earth's motion. He produced interference-fringes between 

 two pencils of light which had traversed paths perpendicular 

 to each other ; but when the apparatus was rotated through a 

 right angle, so that the difference would be reversed, the expected 

 displacement of the fringes could not be perceived. This result 

 was regarded by Michelson himself as a vindication of Stokes's 

 theory^ in which the aether in the neighbourhood of the 

 dearth is supposed to be set in motion. Lorentzj), however, 

 showed that the quantity to be measured had only half the 

 value supposed by Michelson, and suggested that the negative 

 result of the experiment might be explained by that combina- 

 tion of Fresnel's and Stokes's theories which was developed in 

 his own memoirIF ; since, if the velocity of the aether near the 

 earth were (say) half the earth's velocity, the displacement of 

 Michelson's fringes would be insensible. 



* Phil. Mag. iv. (1902), p. 215. 



t The constant of aberration is the ratio of the earth's orbital velocity to the 

 velocity of light ; cf. supra, p. 100. 



Amer. Journ. Sci. xxii (1881), p. 20. His method was afterwards improved : 

 cf. Michelson and Morley, Amer. Journ. Sci. xxxiv (1887), p. 333; Phil. Mag. 

 xxiv (1887), p. 449. 



Cf. p. 411. 



|| Arch. Xeerl. xxi (1886), p. 103. On the Micbelson-Morley experiment cf. 

 also Hicks, Phil. Mag. iii (1902), p. 9. 



U Cf. p. 413. 



2 E 



