in Media of every degree of Transparency. 117 
The discrepancy is most easily shown in the most simple 
case, when the medium is isotropic and perfectly transparent, 
and @ reduces to a numerical quantity. The square of the 
velocity of plane waves is then equal to er and equation (18) 
would make it independent of the period; that is, would give 
no dispersion of colors. The case is essentially the same in 
transparent bodies which are not isotropic.* 
; case is worse with metals, which are characterized elec- 
trically by great conductivity, and optically by great opacity. 
In their papers cited above, Lorentz and Rayleigh have 
tion, therefore, is essentially the same as that which Lord Ray- 
leigh 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. : 
I this points to the same conclusion—that the ordi- 
nary 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 ordi- 
nary theory which is verified by experiment, without including 
that part which is in opposition to observed facts.f 
