PRESIDENT’S ADDRESS. 17 
The ether carried along by a wave of light must be an exceedingly 
small part of the volume through which the wave is spread. Parts of 
this volume are in motion, but by far the greater part is at rest; thus 
in the wave front there cannot be uniformity, at some parts the ether 
is moving, at others it is at rest—in other words, the wave front must 
be more analogous to bright specks on a dark ground than to a uniformly 
illuminated surface. 
The place where the density of the ether carried along by an 
electric field rises to its highest value is close to a corpuscle, for 
round the corpuscles are by far the strongest electric fields of which we 
have any knowledge. We know the mass of the corpuscle, we know 
from Kaufmann’s experiments that this arises entirely from the electric 
charge, and is therefore due to the ether carried along with the corpuscle 
by the lines of force attached to it. 
A simple calculation shows that one-half of this mass is contained in 
a volume seven times that of a corpuscle. Since we know the volume 
of the corpuscle as well as the mass, we can calculate the density of the 
ether attached to the corpuscle; doing so, we find it amounts to the 
prodigious value of about 5 x 10?°, or about 2,000 million times that 
of lead. Sir Oliver Lodge, by somewhat different considerations, has 
arrived at a value of the same order of magnitude. 
Thus around the corpuscle ether must have an extravagant density : 
whether the density is as great as this in other places depends upon 
whether the ether is compressible or not. If it is compressible, then it 
may be condensed round the corpuscles, and there have an abnormally 
great density ; if it is not compressible, then the density in free space 
cannot be less than the number I have just mentioned. 
With respect to this point we must remember that the forces acting 
on the ether close to the corpuscle are prodigious. If the ether were, for 
example, an ideal gas whose density increased in proportion to the 
pressure, however great the pressure might be, then if, when exposed to 
the pressures which exist in some directions close to the corpuscle, it had 
the density stated above, its density under atmospheric pressure would 
only be about 8 x 10~", or a cubic kilometre would havea mass less than 
a gramme; so that instead of being almost incomparably denser than 
lead, it would be almost incomparably rarer than the lightest gas. 
I do not know at present of any effect which would enable us to 
determine whether ether is compressible or not. And although at first 
sight the idea that we are immersed in a medium almost infinitely 
denser than lead might seem inconceivable, it is not so if we remember 
that in all probability matter is composed mainly of holes. We may, 
in fact, regard matter as possessing a bird-cage kind of structure 
in which the volume of the ether disturbed by the wires when 
the structure is moved is infinitesimal in comparison with the volume 
enclosed by them. If we do this, no difficulty arises from the great 
1909, c 
