ope, i.e. if m is
the mass, e the charge on a corpuscle, v the greatest velocity of
projection, until Ve = 1/2mv^2. The values found for V by different
observers are not very consistent. Lenard found that V for aluminium was
about 3 volts and for platinum 2. Millikan and Winchester (_Phil. Mag._,
July 1907) found for aluminium V = .738. The apparatus used by them was
so complex that the interpretation of their results is difficult.

An extremely interesting fact discovered by Lenard is that the velocity
with which the corpuscles are emitted from the metal is independent of
the intensity of the incident light. The quantity of corpuscles
increases with the intensity, but the velocity of the individual
corpuscles does not. It is worthy of notice that in other cases when
negative corpuscles are emitted from metals, as for example when the
metals are exposed to cathode rays, Canal-strahlen, or Rontgen rays, the
velocity of the emitted corpuscles is independent of the intensity of
the primary radiation which excites them. The velocity is not, however,
independent of the nature of the primary rays. Thus when light is used
to produce the emission of corpuscles the velocity, as Ladenburg has
shown, depends on the wave length of the light, increasing as the wave
length diminishes. The velocity of corpuscles emitted under the action
of cathode rays is greater than that of those ejected by light, while
the incidence of Rontgen rays produces the emission of corpuscles moving
much more rapidly than those in the cases already mentioned, and the
harder the primary rays the greater is the velocity of the corpuscles.

The importance of the fact that the velocity and therefore the energy of
the corpuscles emitted from the metal is independent of the intensity of
the incident light can hardly be overestimated. It raises the most
fundamental questions as to the nature of light and the constitution of
the molecules. What is the source of the energy possessed by these
corpuscles? Is it the light, or in the stores of internal energy
possessed by the molecule? Let us follow the consequences of supposing
that the energy comes from the light. Then, since the energy is
independent of the intensity of the light, the electric forces which
liberate the corpuscles must also be independent of that intensity. But
this cannot be the case if, as is usually assumed in the electromagnetic
theory, the wave front consists of a uniform distribution of electric