FEBRUARY 23, 1912] 
tion to the principle is the expressed con- 
dition that in the system of reference the 
laws of nature hold in a definite and 
mathematically simple form. There is no 
warrant in the past history of physics for 
the adoption of such a postulate as that. 
Surely the history of the discovery of the 
so-called secondary laws of physics, such as 
Boyle’s law, the laws of friction, the laws 
of polarization and of absorption of light, 
the laws of magnetization, and many 
others, will bear out the statement that in 
very many eases the first enunciation of 
the law is in a definite and mathematically 
simple form, and that further knowledge 
shows that this form is only a first ap- 
proximation to the truth. Hven in the 
ease of such laws as the law of gravitation, 
or of electrical attraction and repulsion 
from which we have not yet detected any 
deviation, does any one dare to say that 
they are universally true for all bodies and 
at all distances? Can we even feel sure 
that Maxwell’s electromagnetic equations 
hold true with absolute exactness? They 
need supplementing when they are applied 
to material bodies. Can we be sure that 
they hold without modification, in rapidly 
moving bodies, or at extremely minute dis- 
tances in free space. Or, from another 
point of view, admitting that the object of 
physical study is to reduce the description 
of natural phenomena to a set of simple 
laws, have we a right to assume that, in 
our analysis of the structure of matter and 
of the luminiferous medium, we have as 
yet reached the ultimate model in which 
such simple laws will be operative? ‘The 
answer to this question must be a negative 
one. Yet it is surely true that if it were 
not for this demand of simplicity, immedi- 
ately attainable and at present expressed 
in the electromagnetic equations, the chief 
incentive to the development of the theory 
of relativity would be wanting. 
SCIENCE 
289 
But this is not the heart of the matter. 
With the principle of relativity as a basal 
postulate, not expressing our present ina- 
bility, but rather the hopelessness of any 
attempt to obtain ability, a complete de- 
scription has been given of the phenomena 
now known to physicists, at least in the 
domains of mechanics, light and electricity. 
The difficulty which I find in accepting the 
principle, with the universality that is 
predicated of it, is that it does so much 
more than this. 
The theories of J. J. Thomson and of 
Lorentz made physicists familiar with the 
notion of electrical mass, exhibited by the 
variability of the mass of a moving 
charged body, or by the apparent variable 
inertia of a moving charge expressed as a 
function of its velocity, and further with 
the notion that as the velocity of the charge 
approaches the velocity of light the mag- 
nitude of the electrical mass approaches 
infinity, so that the velocity of an electrical 
charge, of an electron, and therefore pre- 
sumably of matter, if it is entirely electric- 
al in its structure, can never surpass the 
velocity of light. In these theories this re- 
markable conclusion was explained by the 
interaction between the moving charge and 
the ether. In the theory of relativity the 
same conclusion is reached as the conse- 
quence of a purely kinematical theorem, 
giving the rule for the addition of veloci- 
ties, and not only does it hold for real 
moving charges, but for any action what- 
ever which is conceivably transmitted 
through space. In particular this finite 
velocity of transmission must be ascribed 
to gravitational action. Now the Newton- 
ian theory of gravitation assumes a prac- 
tically infinite velocity of transmission of 
gravitational action, and astronomical 
observations have never given any war- 
rant for the belief that its velocity of 
transmission is even of the order of mag- 
