M. I, Pupin—Studies in the Electro-magnetic Theory. 335 
phenomena, as, for instance, dispersion, absorption, rotation of 
the plane of polarization, etc. In other words, it fails to give 
a satisfactory general relation between optical and electro-mag- 
netic constants without resorting to additional hypotheses. 
This failure is not confined to the electro-magnetic theory ; 
all optical theories labor under the same difficulty. There is, 
however, one distinct and important advantage which the 
electro-magnetic theory has over other optical theories. It is 
the definiteness of its premises.. For whereas the dynamical 
theories of light must start with the material attributes, den- 
sity and elasticity, of an unknown substance, the ether, which - 
possibly may not resemble ordinary matter and whose material 
attributes, if it has them at all, seem to be entirely outside of 
the possibility of direct experimental test, the electro-mag- 
netic theory endows the same substance with no other attri- 
butes excepting those contained in Maxwell’s first and second 
law of electro-dynamics and the law of flux, and it proves the 
actual existence of these attributes by direct experiment cover- 
ing a large region of electro-magnetic phenomena, the region 
of the Hertzian waves. It is, therefore, perfectly sure of its 
round in one extensive region of ether manifestations. 
The limitations of the Maxwellian hypotheses are also very 
clear from the character of the experiments which guided the 
formation of these hypotheses. These experiments were per- 
formed with constant or very slowly-varying forces, hence the 
obvious probability that Maxwell’s hypotheses need a modifica- 
tion in the case of electro-magnetic phenomena involving the 
operation of rapidly-varying forces; in other words, Maxwell’s 
electro-magnetic theory must be extended so as to include the 
periodicity of its fundamental vectors before it can become a 
_ satisfactory optical theory. 
The first and the second law of Maxwell’s electro-dynamics 
in the form stated above contain no mention of the physical 
constants of the field. If our experience with optical phe- 
nomena can be relied upon to guide us in our attempt to 
extend the electro-magnetic theory, then we should naturally 
infer that the element of periodicity can enter into Maxwell’s 
electro-magnetic theory through the physical constants, only, of 
the field, and therefore through the law of flux and not 
through the two fundamental laws, which, in my opinion, do 
not contain these constants explicitly. It is, therefore, the law 
of flux which in all probability must be extended so as to 
include the periodicity of the impressed forces, if Maxwell’s 
electro-magnetic theory is to become a theory of light. A 
careful analysis of the several satisfactory electro-magnetic 
theories of dispersion and absorption and of the magnetic rota- 
tion of the plane of polarization supports this conclusion. I 
