1893-94.] Prof. Tait on Clerk- MaxivelVs Ec[uations. 
213 
Note on the Antecedents of Clerk-Maxwell’s Electro- 
dynamical-Wave-Equations. By Prof. Tait. 
(Read April 2, 1894.) 
The first obvious difficulty which presents itself, in trying to 
derive Clerk-Maxwell’s equations from those of the elastic-solid 
theory, appears in the fact that the latter, being linear, do not 
impose any relations among simultaneous disturbances. Thus, for 
instance, they indicate no reason for the associated disturbances 
which, in Maxwell’s theory, constitute a ray of polarised light. 
Hence it appears that we must look on the vectors of electric and 
magnetic force, if they are to be accounted for on ordinary dyna- 
mical principles, as being necessary concomitants, qualities, or 
characteristics of one and the same vector-disturbance of the ether, 
and not themselves primarily disturbances. From this point of 
view the disturbance, in itself, does not correspond to light, and 
may perhaps not affect any of our senses. And the very form of 
the elastic equation at once suggests any number of sets of two 
concomitants of the desired nature, which are found to be related 
to one another in the way required by Maxwell’s equations. 
For the moment, as sufficiently illustrating the essential point 
of the above remarks, I confine myself to disturbances, in the free 
ether, such as do not involve change of volume. The elastic equation 
is 
e= 
with the limiting condition 
SV0=O . * 
[Had not this condition been imposed, the dynamical equation 
would have involved, on the right, the additional term 
(a2_Z,2)vSV0.] 
* Stokes “On the Dynamical Theory of Diffraction,” Caml. Phil. Trans., 
ix. (1849). 
