force without structure, for in this case the magnitude of the electric
force is proportional to the square root of the intensity. On the
emission theory of light a difficulty of this kind would not arise, for
on that theory the energy in a luminiferous particle remains constant as
the particle pursues its flight through space. Thus any process which a
single particle is able to effect by virtue of its energy will be done
just as well a thousand miles away from the source of light as at the
source itself, though of course in a given space there will not be
nearly so many particles to do this process far from the source as there
are close in. Thus, if one of the particles when it struck against a
piece of metal caused the ejection of a corpuscle with a given velocity,
the velocity of emission would not depend on the intensity of the light.
There does not seem any reason for believing that the electromagnetic
theory is inconsistent with the idea that on this theory, as on the
emission theory, the energy in the light wave may instead of being
uniformly distributed through space be concentrated in bundles which
occupy only a small fraction of the volume traversed by the light, and
that as the wave travels out the bundles get farther apart, the energy
in each remaining undiminished. Some such view of the structure of light
seems to be required to account for the fact that when a plate of metal
is struck by a wave of ultra-violet light, it would take years before
the corpuscles emitted from the metal would equal in number the
molecules on the surface of the metal plate, and yet on the ordinary
theory of light each one of these is without interruption exposed to the
action of the light. The fact discovered by E. Ladenburg (_Verh. d.
deutsch. physik. Ges._ 9, p. 504) that the velocity with which the
corpuscles are emitted depends on the wave length of the light suggests
that the energy in each bundle depends upon the wave length and
increases as the wave length diminishes.

These considerations illustrate the evidence afforded by photo-electric
effects on the nature of light; these effects may also have a deep
significance with regard to the structure of matter. The fact that the
energy of the individual corpuscles is independent of the intensity of
the light might be explained by the hypothesis that the energy of the
corpuscles does not come from the light but from the energy stored up in
the molecules of the metal exposed to