OF EVERY DEGREE OF TRANSPARENCY. 221 



period; that is, would give no dispersion of colors. The case is 

 essentially the same in transparent bodies which are not isotropic.* 



The case is worse with metals, which are characterized electrically 

 by great conductivity, and optically by great opacity. In their 

 papers cited above, Lorentz and Rayleigh have observed that the 

 experiments of Jamin on the reflection of light from metallic surfaces 

 would often require, as ordinarily interpreted on the electromagnetic 

 theory, a negative value for the inductive capacity of the metal. 

 This would imply that the electrical equilibrium in the metal is 

 unstable. The objection, therefore, is essentially the same as that 

 which Lord Rayleigh had previously made to Cauchy's theory of 

 metallic reflection, viz., that the apparent mechanical explanation 

 of the phenomena is illusory, since the numerical values given by 

 experiment as interpreted on Cauchy's theory would involve an 

 unstable equilibrium of the ether in the metal. t 



13. All this points to the same conclusion that the ordinary view 

 of the phenomena is inadequate. The object of this paper will be 

 accomplished, if it has been made clear how a point of view more in 

 accordance with what we know of the molecular constitution of 

 bodies will give that part of the ordinary theory which is verified by 

 experiment, without including that part which is in opposition to 

 observed facts.! 



* See note to the first paper of Lorentz, cited above, Schlomilch, vol. xxii, p. 23. 



t See Phil. Mag. (4), vol. xliii (1872), p. 321. 



J The consideration of the processes which we may suppose to take place in the 

 smallest parts of a body through which light is transmitted, farther than is necessary to 

 establish the general equation given above, is foreign to the design of this paper. Yet a 

 word may be added with respect to the difficulties signalized in the ordinary form of the 

 theory. The comparatively simple case of a perfectly transparent body has been 

 examined more in detail in one of the papers already cited, where there is given an 

 explanation of the dispersion of colors from the point of view of this paper. It is there 

 shown that the effect of the non-homogeneity of the body in its smallest parts is to add 

 a term to the expression for the kinetic energy of electrical waves, which for an isotropic 

 body may be roughly described as similar to that which would be required if the 

 electricity had a certain mass or inertia. (See especially 7, 9 and 12, [this volume 

 pages 185 ff.]) The same must be true of media of any degree of opacity. Now the 

 difficulty with the optical properties of the metals is that the real part of 6 (or 6" 1 ) 

 is in some cases negative. This implies that at a moment of greatest displacement 

 the electromotive force is in the direction opposite to the displacement, instead of 

 having the same direction, as in transparent isotropic bodies. Now a certain part of 

 the electromotive force must be required to oppose the apparent inertia, and another 

 part to oppose the electrical elasticity of the medium. These parts of the force must 

 have opposite directions. In transparent bodies the latter part is by far the greater. But 

 it need not surprise us that the former should be the greater in some metals. 



It has been remarked by Lorentz that the difficulty with respect to metals would be 

 in a measure relieved if we should suppose electricity to have the property of inertia. 

 (See 11 of his third paper, Schlomilch's Zeitschrift, vol. xxiii, p. 208.) But a supposi- 

 tion of this kind, taken literally, would involve a dispersion of colors in vacuo, and still 

 be inadequate, as Lorentz remarks, to explain the phenomena observed in metals. 



