282 Maxwell. 



calculated by considering the mechanical force required in 

 order to increase the distance between the plates of a condenser, 

 so as to enlarge the field comprised between them. The result 

 is that the energy per unit volume of the dielectric is fE /2 /87r, 

 where c denotes the specific inductive capacity of the dielectric 

 and E' denotes the electric force, measured in terms of the 

 electrostatic unit : if E denotes the electric force expressed in 

 terms of the electrodynamic units used in the present investi- 

 gation, we have E = cE', where c denotes the constant which* 

 occurs in transformations of this kind. The energy is therefore 

 fcE 2 /87TC 2 per unit volume. Comparing this with the expression 

 for the energy in terms of E and D, we have 



D 



and therefore the constant Ci has the value ct*. Thus the 

 result is obtained that the velocity of propagation of dis- 

 turbances in Maxwell's medium is ce~, where denotes the 

 specific inductive capacity and c denotes the velocity for which 

 Kohlrausch and Weber had foundf the value 3*1 x 10 10 cm./sec. 

 Now by this time the velocity of light was known, not only 

 from the astronomical observations of aberration and of Jupiter's 

 satellites, but also by direct terrestrial experiments. In 1849 

 Hippolyte Louis FizeauJ had ' determined it by rotating a 

 toothed wheel so rapidly that a beam of light transmitted 

 through the gap between two teeth and reflected back from a 

 mirror was eclipsed by one of the teeth on its return journey. 

 The velocity of light was calculated from the dimensions and 

 angular velocity of the wheel and the distance of the mirror ; 

 the result being 3*15 x 10 10 cm. /sec. 



* Cf. pp. 227, 259. | Cf. p. 260. 



| Comptes Rendus, xxix (1849), p. 90. A determination made by Cornu in 

 1874 was on this principle. 



A different experimental method was employed in 1862 hy Leon Foucault 

 (Comptes Rendus, Iv, pp. 501, 792) ; in this a ray from an origin was reflected 

 by a revolving mirror M to a fixed mirror, and so reflected back to J/, and again 

 to O. It is evident that the returning ray ?dO must be deviated by twice the 

 angle through which M turns while the light passes from M to the fixed mirror 

 and back. The value thus obtained by Foucault for the velocity of light was 



