Principle of Relativity. 317 
o£ observation, leaving it to future endeavours to find a 
physical interpretation of this method. No doubt this way of 
proceeding implies a certain resignation. But in view of the 
failure of the electromagnetic theories advanced as yet, it 
seems the safest road to follow. 
My method rests on ths following principles and de- 
finitions : — 
(1) The validity of the Maxwellian " differential " equations 
associated with ordinary kinematics. 
(2) The distinction, for the mere sake of calculation, between 
active and passive systems, whenever forces are concerned 
which two electromagnetic systems in uniform relative motion 
exert on each other. 
(3) The prescription : Whenever the force on one of the 
two systems due to the other is required, choose the former 
as the passive one and calculate the force on it exactly as in 
the original Maxwellian theory, as though it were " at rest in 
the ?ether " and the other " moving through the aether." 
I have proved (I. c.) that this prescription is consistent, 
i. e. that the forces thus calculated are identical whichever of 
the two systems is chosen as passive. This implies the prin- 
ciple of relativity of motion for the systems considered. 
Mr. Cunningham will admit that this method of calculating 
is perfectly definite, and by a careful comparison with the 
Lorentz- Einstein principle he will convince himself that 
the two principles are essentially different. The remark of 
Mr. Cunningham that my principle is identical with that of 
Einstein except that I omit the transformation of time and 
space coordinates, appears untenable also from the following 
consideration. Evidently, according to Mr. Cunningham, a 
transformation of the forces experienced by a moving electron 
in a condenser field and in the field of an electromagnet as 
calculated by me (I. c.) should yield the expressions given 
by Einstein and Lorentz. But an inspection of my equations 
proves the impossibility of such a transformation. In fact 
no other known theory of electromagnetism leads to these 
forces. 
As I employ the ordinary kinematics, only a spherical 
electron will jit in my theory. Mr. Cunningham has overlooked 
this circumstance. 
However, it does not follow that the same formula as 
Abraham's should be applied, as this formula is connected 
with the expression of the field energy, and the latter is 
introduced by me as a special hypothesis (I. c. p. 418). 
Whereas it will be impossible to point to any discrepancy 
Phil May. S. 6. Vol. 15. No. 87. March 1908. Z 
