JUNE 11, 1914] 
consequence that this standard cannot be a unique 
one. When, for example, Lord Rayleigh conceived 
and carried out in 1902 an experiment in which 
he sought to find evidence of double refraction in 
a plate of glass owing to its motion through the 
ether, Sir Joseph Larmor gave it as his opinion 
that the negative result was to be expected on 
theoretical grounds. 
It may be taken indeed as proved that in so 
far as matter is electrically constituted, the form 
of the equations which embody the theory is such 
that effects due to the motion of bodies as a whole 
through the ether must always be concealed. 
But is matter of purely electromagnetic con- 
stitution? Are existing theories able to give a 
complete account of those phenomena which have 
actually been experimentally investigated ? 
The classical experiment of Michelson and 
Morley may be taken as an example on which 
to test these questions. It is generally admitted 
that this experiment shows that we cannot 
detect a difference in the velocity of light rela- 
tive to the earth in two directions at right angles 
one of which may be thought of as_ parallel 
and the other perpendicular to the motion of the 
earth through the ether. Such a difference must 
exist if light is thought of as being propagated 
with the same velocity in all directions relative to 
the ether. 
The only suggestion that could be made to 
reconcile the failure of the search for this differ- 
ence with the theory of a stationary ether was 
that of FitzGerald, that the motion of the appa- 
ratus through the ether so modifies its internal 
constitution that it automatically contracts to an 
extent which exactly neutralises the effect which 
would otherwise be observed. It was in the effort 
to give a reason for this contraction that the 
theory that has been referred to was developed. 
But whether we take the presentation given by 
Larmor or Lorentz we find that the general equa- 
tions of the electromagnetic field have to be 
supplemented at some point by a hypothesis as to 
the nature of the electrons which are the ele- 
mentary constituents of matter, in order to make 
the scheme sufficient to determine the way in 
which they will move. Now the length of a 
body, thought of as constituted by electrons, de- 
pends upon the motions of those electrons. If we 
are to think of any piece of matter whatever as 
contracting according to FitzGerald’s hypothesis, 
we are bound to think of the paths of the electrons 
within the body as being modified in some corre- 
sponding way. Thus the hypotheses that may be 
adopted as to the nature of the electron are not 
arbitrary, but must be such as will lead to the 
contraction hypothesis as a consequence. 
Similarly, if we consider the experiment of 
Rayleigh referred to above, the refracting proper- 
ties of glass are conceived to be due to the licht 
waves falling upon electrons which have inertia 
and which have to be moved by the electrical 
forces produced by the light. If we were to 
assume that the electrons have a definite mass in 
the Newtonian sense, then Rayleigh’s expectation 
NG 2328, VOL. Q3 
NATURE 
379 
of a double refraction when the glass is moving 
would be justified. Lorentz is able, however, by 
assuming among other things that the electron is 
a spherical nucleus which itself is subject to the 
FitzGerald contraction, to extend his argument to 
cover the null result of this experiment. But the 
special assumptions which he makes were all made 
,; with an eye towards the result, namely, the failure 
of experiment to give a positive evidence of motion 
through the ether. They were hypotheses ad 
hoc, and to that extent they were really, though 
the name had not been invented, applications of 
the principle of relativity. It cannot be shown 
from the form of the general equations of the 
electromagnetic field alone that null effects are to 
be expected, for the experimental results most 
certainly extend into regions where these equa- 
tions are insufficient; they do not cover, for in- 
stance, the whole theory of refraction, of con- 
duction of electricity, or of the exterior configura- 
tion of a given body. 
It is for this reason that the hypothesis that the 
fact of motion relative to the aether must be for ever 
concealed, becomes of importance as a general and 
independent principle. It becomes a criterion and a 
guide, for example, as to the form that is to be 
chosen for the constitutive relations which connect 
the electric force and displacement, the magnetic 
force and induction, and the current in moving 
bodies. It leads us to the conclusion that the 
Newtonian conception of a constant mass needs 
some revision if the hypothesis is true, and at 
this point comes into touch with the experiments 
on the variation of the apparent inertia of a 
negative electron with its velocity, and in fact 
is here confirmed. 
But although experiment suggested and has so 
far confirmed the validity of the hypothesis, yet 
two serious objections are raised against it. The 
first is that it conflicts with our simplest ideas as 
to the measurement of space and time, and the 
second is that it abolishes the ether as a unique 
and objective medium, the seat of all electrical 
activity. In a succeeding article an attempt will 
be made to indicate what position in regard to 
these two very important points the adoption of 
the hypothesis requires us to take. 
E. CUNNINGHAM. 
DRO REV NOEDS (GREEN, UPR AS. 
HE announcement of the death of Dr. Rey- 
nolds Green, on June 3, will have been re- 
ceived with unfeigned regret by all his scientific 
fellow-workers, whether botanists or physiolo- 
gists. For those who, like myself, have known 
him throughout his career with a considerable 
degree of intimacy, regret amounts to a deep 
sense of personal loss. It is some consolation to 
me to have this opportunity of writing a few words 
in appreciation of him who was so closely associated 
with me first as pupil, then as collaborator, always 
as friend. 
Joseph Reynolds Green came up to Cambridge 
in t880 as a scholar of Trinity College, in which 
