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 FitzGerald contraction of matter as it moves through the aether might conceivably be supposed to affect in some way the optical properties of the moving natter; for instance, transparent substances might become doubly refracting. Experiments designed to test this supposition were performed by Lord Rayleigh in 1902,[1] and by D. B. Brace in 1904[2]; but no double refraction comparable with the proportion (w/c)2 of the single refraction could be detected. The FitzGerald 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,[3] 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 Larmor[4] extended the analysis so as to include small quantities of the second order, and thereby discovered a remarkable connexion between the equations of transformation and the equations which represent FitzGerald's con-
