Closing Years of the Nineteenth Century. 439 



constraint, is independent of the orientation of the plates with 

 respect to the direction of the terrestrial motion. 



It may be remarked that the existence of the couple, had 

 it been observed, would have demonstrated the possibility of 

 drawing on the energy of the earth's motion for purposes of 

 terrestrial utility. 



The Fitz Gerald contraction of matter as it moves through 

 the aether might conceivably be supposed to affect in some 

 way the optical properties of the moving matter; for in- 

 stance, transparent substances might become doubly refracting. 

 Experiments designed to test this supposition were per- 

 formed by Lord Eayieigh in 1902,* and by D. B. Brace in 

 1904t ; but no double refraction comparable with the propor- 

 tion (w/cf of the single refraction could be detected. The 

 Fitz Gerald contraction of a material body cannot therefore be 

 of the same nature as the contraction which would be produced 

 in the body by pressure, but must be accompanied by such 

 concomitant changes in the relations of the molecules to the 

 aether that an isotropic substance does not lose its simply 

 refracting character. 



By this time, indeed, the hypothesis of contraction, which 

 originally had no direct connexion with electric theory, had 

 assumed a new aspect. Lorentz, as we have seen,J had 

 obtained the equations of a moving electric system by 

 applying a transformation to the fundamental equations of 

 the aether. In the original form of this transformation, 

 quantities of higher order than the first in w/c were neglected. 

 But in 1900 Larmorg extended the analysis so as to include 

 small quantities of the second order, and thereby discovered a 

 remarkable connexion between the equations of transforma- 

 tion and the equations which represent Fitz Gerald's con- 



Phil. Mag. iv (1902), p. 678. 

 t Phil. Mag. vii (1904), p. 317. 



+ Cf. p. 434. Cf. also Lorentz, Proc. Amsterdam Acad. (English ed.), i (1899), 

 p. 427. 



Larmor, Aether and Matter, p. 173. 



