288 
venience of being able to apply Maxwell’s 
equations of the electromagnetie field with- 
out change of form to a system referred to 
moving axes. It is not needed to explain 
many of the remarkable results obtained 
by Fizeau, by Mascart and by Brace, in 
the field of experimental optics, which to a 
first inspection seem to show that the earth 
and the medium around it through which 
light passes are relatively at rest, but which 
a closer study by Lorentz and others shows 
may be compatible with a reasonable 
theory of the structure of matter and the 
hypothesis that the luminiferous medium 
is at rest. It is also not needed to explain 
the dependence of the path of an electron 
in a field of crossed electric and magnetic 
forces upon its velocity, as exhibited in the 
beautiful experiments of Kauffmann and 
of Bucherer, for other theories in which 
the principle is not used lead to expres- 
sions for the path which, fcr the present at 
least, are in as good accord with observa- 
tion as those which are deduced by the aid 
of the principle of relativity. 
There are two ways of presenting the 
principle of relativity. In the first way 
the principle is stated as a direct inductive 
conclusion from the experiment of Michel- 
son and Morley, and asserts that so far as 
a conelusion can be drawn from that ex- 
periment and the others which have been 
tried to test the matter, there is no way by 
which the relative motion of the earth and 
the luminiferous medium can be deter- 
mined from observations made on the pas- 
sage of light when the source of light and 
the observer are moving with the earth. 
As thus presented the principle holds out 
as the object of future study the construc- 
tion of a suitable theory of the structure of 
matter and of the luminiferous medium to 
account for this fundamental experiment 
as well as for all other known truths in 
the domains of light and electricity. If 
SCIENCE 
[N.S. Von. XXXV. No. 895 
this theory is expressed in terms of the 
Lorentz transformation, and thus shows a 
dependence of the measure of time and the 
measure of length upon the velocity of the 
system in which the observer is placed, it 
will further be the object of inquiry to 
construct a theory of the relations between 
the material of the system and the lumi- 
niferous medium which will account for 
the change in the units of length and in 
the motions of bodies by which the unit of 
time is determined. When I say to ac- 
count for, I mean to describe in terms of 
force, time and space, as we conceive 
those notions in our every-day experience, 
and as we use them in our ordinary phys- 
ical work, so that the description when 
apprehended will be reduced to the lowest 
terms in which our thought about the uni- 
verse can be expressed. Such a descrip- 
tion is, as I view it, a real explanation, 
and surely it is not yet time to say that 
such an explanation is impossible. 
The other way of presenting the prin- 
ciple of relativity consists in laying down 
as a fundamental postulate a general 
proposition expressing the hopelessness of 
any attempt to settle the question raised 
by the experiment of Michelson and Morley 
by any theory of the structure of the uni- 
verse. This postulate sometimes assumes 
a formidable aspect, and involves more 
than the mere postulate of relativity. Thus 
Laue says: 
The principle of relativity asserts that from the 
totality of natural phenomena we may, with con- 
tinually increasing approximation, determine a 
system of reference, x, y, 2, t, in which the laws 
of nature hold in a definite and mathematically 
simple form. This system of reference is by no 
means uniquely determined by the phenomena. 
There is rather a triple infinity of equally admis- 
sible systems, which move relatively to one another 
with uniform velocities. 
The feature of this enunciation of the 
principle to which I referred as an addi- 
