448 Prof. C. V. Raman and Dr. N. K. Sethi on the 



with the experiments o£ Michelson and Morley *, which 

 seem to contradict this fixity of the external sether, has been 

 explained away by FitzGerald and Lorentz as being due to 

 an inevitable change in the dimensions of the apparatus on 

 account of its motion with the earth, an explanation very 

 considerably simplified and made almost self-evident by the 

 advent of the Relativity Principle f . 



But so far as the second part is concerned,, the only 

 evidence of a positive character in favour of this law of 

 sether drift was furnished by the celebrated experiments of 

 Fizeau, in which a delicate interference method 'was em- 

 ployed to detect and measure a change in the velocity of 

 light on account of a velocity of about 7 metres per second 

 of a column of water through which it was made to travel. 

 This experiment was repeated by Michelson and Morley J in 

 1886 with improved apparatus, and it demonstrated the 

 surprisingly good agreement with theory. But the appear- 

 ance of the elaborate electron theory of Lorentz made this 

 agreement much less brilliant ; for he showed that in dis- 

 persive media the convection coefficient was not 1 ^, but 



1 T 1 f^ 1 



1 -^r« This necessitated a more careful repetition 



/ju" fju d L 

 of the experiment which has recently been accomplished by 

 Zeeman §, and the result is decidedly in favour of the new 

 theory. The subject has been further followed up by him, 

 and he has succeeded in overcoming the enormous experi- 

 mental difficulties, and has actually determined for glass and 

 quartz not only the Fresnel coefficient, but also the Lorentz 

 correction for dispersion. 



In view of this recent work and the accuracy which has 

 been attained in the measurements, it is hardly necessary to 

 refer to the work of Sir J. J. Thomson ||, which led to the 

 result that an electromagnetic wave inside a moving body 

 should drift with half the velocity of that body. In support 

 of this, it was argued that the substances for which FresneFs 

 law was actually verified in a positive manner happened to 



be such that 1— -% was in their case very nearly equal 



f 1 ■ 



^ i_ Rut even before the work of Zeeman, it was quite 



evident from the negative results of the experiments with 



* P il. Mag. 1887, p. 449. 



f Cunningham, ' Relativity and Electron Theory,' p. 34. 

 X Amer. Jouvn. of Science, xxxi. p. 377 (1886). 



§ K. Akad. Amst. Proc. xvii. p. 445 (1914); xviii. p. 398 (1915); 

 xxii. pt>. 462 & 512 (1920). 

 || Phil. Mag. April 1880. 



