334 M. I. Pupin—Studies in the Electro-magnetic Theory. 
will be pointed out presently, actually does just as well, in many 
instances, as the real thing, but there are considerations, to be 
discussed presently, which seem to speak very forcibly against 
the equivalence between this adulteration and the real thing. 
The main thread of the discussion can now be taken up 
again. Combining the two fundamental laws with the law of 
flux, the well known relations of which the following are types 
are easily obtained: 
a A*X 
pM 
For all non-absorptive isotropic insulators we shall have 
ee one 
FT een qe a 
da Von 
ge lighs ea 
These equations describe the source of indirect experimental 
evidences of the Maxwellian theory. They constitute the second 
part of this theory, and they show better than any words can 
_ the important function which the physical constants of the field 
perform in our modern views of electro-magnetic phenomena. 
It was reserved for Hertz and his followers to reveal by actual 
experiment the wealth of this source and to demonstrate 
beyond all reasonable doubt that long electro-magnetic waves 
are propagated in perfect accordance with the laws stated in 
these equations. These laws are, within certain limits which 
will be mentioned presently, a perfect analogy to the laws of 
propagation of luminous waves. Hence experimental results 
obtained from electro-magnetic phenomena involving the opera- 
tion of forces whose period is not shorter than the shortest 
period of the Hertzian waves, give a decisive victory for the 
hypotheses which underlie the Maxwellian electro-magnetic 
theory. 
But this theory is much broader. It states that, probably, 
hight is an electro-magnetic phenomenon. It fails, however, to 
give a satisfactory account of some of the most striking optical 
