AND THE THEORY OF A QUASI-LABILE ETHER. 245 



light. This is strikingly true of the metals. But the analogy does 

 not stop here. To fix our ideas, let us consider the case of an 

 isotropic body and circularly polarized light, which is geometrically 

 the simplest case although its analytical expression is not so simple 

 as that of plane-polarized light. The displacement at any point 

 may be symbolized by the rotation of a point in a circle. The 

 external force necessary to maintain the displacement is represented 

 by n~ 2 $. In transparent bodies, for which n~ 2 is a positive number, 

 the force is radial and in the direction of the displacement, being 

 principally employed in counterbalancing the dielectric elasticity, 

 which tends to diminish the displacement. In a conductor n~ z 

 becomes complex, which indicates a component of the force in the 

 direction of $, that is, tangential to the circle. This is only the 

 analytical expression of the fact above mentioned. But there is 

 another optical peculiarity of metals, which has caused much remark, 

 viz., that the real part of n 2 (and therefore of n~ 2 ) is negative, i.e., 

 the radial component of the force is directed towards the center. 

 This inwardly directed force, which evidently opposes the electro- 

 dynamic induction of the irregular part of the motion, is small 

 compared with the outward force which is found in transparent' 

 bodies, but increases rapidly as the period diminishes. We may say, 

 therefore, that metals exhibit a second optical peculiarity, that the 

 dielectric elasticity is not prominent as in transparent bodies. This 

 is like the electrical behavior of the metals, in which we do not 

 observe any elastic resistance to the motion of electricity. We see, 

 therefore, that the complex indices of metals, both in the real and 

 the imaginary part of their inverse squares, exhibit properties 

 corresponding to the electrical behavior of the metals. 



The case is quite different in the elastic theory. Here the force 

 from outside necessary to maintain in any element of volume the 

 displacement @ is represented by n z & In transparent bodies, 

 therefore, it is directed toward the center. In metals, there is a 

 component in the direction of the motion d, while the radial part of 

 the force changes its direction and is often many times greater than 

 the opposite force in transparent bodies. This indicates that in 

 metals the displacement of the ether is resisted by a strong elastic 

 force, quite enormous compared to anything of the kind in trans- 

 parent bodies, where it indeed exists, but is so small that it has been 

 neglected by most writers except when treating of dispersion. We 

 can make these suppositions, but they do not correspond to 

 anything which we know independently of optical experiment. 



It is evident that the electrical theory of light has a serious rival, 

 in a sense in which, perhaps, one did not exist before the publication 



