78G 
MR. J. LARMOR 0^ A DYNAMICAL THEORY OF 
which represent interaction with a motion of partly irrotational character; and this 
exception is evidenced by the necessity which then arises of taking explicit account 
of incompressibility in order to avoid change from rotational to longitudinal undulation 
in a heterogeneous medium. 
92. The question occurs, how far the form of these functions may be susceptible of 
alteration, so as thereby to amend those points in which the account given by the 
electric theory of light is at variance with observation, for example, in the problem of 
metallic reflexion. The form of the function is derived from the phenomena of 
electrical dissipation when the currents are steady or changing wdth comparative 
slowness; as in other cognate cases, it may be subject to modification when the 
rate of alternation is extremely rapid. But as the elastic quality of the medium is 
assumed to be determined by the components of its rotation, and not at all by 
distortion or compression, it seems natural to infer that the viscous resistance to 
change of the strain is determined in terms of the same quantities and therefore by a 
quadratic function of d/dt [f, g, h). This argument, if granted, will carry with it 
the assertion of Ohm’s law of linear conduction in its general form, though probably 
with co-efficients de]Dending on the period, for disturbances of all periods howmver 
small. 
In the expressions for and W, as given above, the principal axes of the 
seolotropic conductivity are taken to coincide with the principal axes of the 
aeolotropic electric displacement, a simplification which need not generally exist. 
The fact that the electric dissipation-function does not involve the velocities of the 
material system slmws that the forces derived from it are solely electromotive. 
93. It seems clear that viscous terms alone could not possibly in any actual 
medium be so potent as to reduce the real part of the complex index of refraction 
suitable to metallic media to be a negative quantity. Such a state of matters arising 
from purely internal action involves instability ; while on the contrary the general 
influence of viscosity is to improve rather than to diminish the dynamical stability of 
a system. This phenomenon, if indeed it is here properly described, must therefore 
be due to the support and control of some other vibrating system ; an explanation 
which has been proposed is to adopt the views of Young and Sellmeier, and ascribe 
its origin to a near approach between the periods of hydrodynamical vibrations of the 
atoms in the molecule and the simultaneous rotational vibrations of the rnther 
produced by the light waves. A theory like this is however usually held as part of 
the larger view which represents ordinary refraction as the result of sjmchronism of 
periods and consequent absorption in the invisible part of the spectrum ; while, in 
the above, the main part of the refraction is ascribed to defect of elasticity due to 
mobile atomic charges. It seems natural therefore to look for some other explanation 
of the discrepancies between theory and observation in ordinary metallic reflexion; 
and the idea suggests itself that if the opacity near the surface w'ere so great as to 
