192 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
verse is constant. Thus when the body is pushed forward by the 
light some other system must have lost the momentum the body 
acquires, and the only other system available is the wave of light 
falling on the body; hence we conclude that there must have been 
momentum in the wave in the direction in which it is traveling. 
Momentum, however, implies mass in motion. We conclude, then, 
that in the ether through which the wave is moving there is mass 
moving with the velocity of ight. The experiments made on the 
pressure due to light enable us to calculate this mass, and we find 
that in a cubic kilometer of ether carrying light as intense as sun- 
light is at the surface of the earth, the mass moving is only about 
one fifty-millionth of a milligram. We must be careful not to con- 
fuse this with the mass of a cubic kilometer of ether; it is only the 
mass moved when the light passes through it; the vast majority of 
the ether is left undisturbed by the light. Now, on the electro- 
magnetic theory of light, a wave of light may be regarded as made 
up of groups of lines of electric force moving with the velocity of 
hight; and if we take this point of view we can prove that the mass 
of ether per cubic centimeter carried along is proportional to the 
energy possessed by these lines of electric force per cubic centimeter, 
divided by the square of the velocity of ight. But though hnes of 
electric force carry some of the ether along with them as they move, 
the amount so carried, even in the strongest electric fields we can 
produce, is but a minute fraction of the ether in their neighborhood. 
This is proved by an experiment made by Sir Oliver Lodge in 
which light was made to travel through an electric field in rapid 
motion. If the electric field had carried the whole of the ether 
with it, the velocity of the light would have been increased by the 
velocity of the electric field. As a matter of fact no increase what- 
ever could be detected, though it would have been registered if it 
had amounted to one-thousandth part of that of the field. 
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 can not 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 elec- 
tric charge, and is therefore due to the ether carried along with the 
corpuscle by the lines of force attached to it. 
