by Ultra-Violet Light, etc. 
421 
interstices between these regions and not by the regions them- 
selves. That in fact the front of the disturbance corresponding to 
the Rontgen rays is not a surface of uniform illumination but 
resembles rather a series of bright specks on a dark ground. We 
can draw similar conclusions about the nature of light from the 
study of the effects of ultra-violet light either on the ioniza- 
tion of gases or on the emission of corpuscles from metal surfaces 
on which the light falls. The existence of the structure in the 
wave front implies that the ether through which the light is 
travelling has also a structure. We shall for the sake of definite- 
ness make a special assumption as to the nature of this structure, 
and suppose that the ether has disseminated through it discrete 
lines of electric force and that these are in a state of tension and 
that light consists of transverse vibrations, Rontgen rays of pulses, 
travelling along these lines. Thus the energy travelling outwards 
with the wave is not spread uniformly over the wave front, but is 
concentrated on those parts of the front where the pulses are 
travelling along the lines of force ; these parts correspond to the 
bright specks, the rest to the dark ground. There will not 
necessarily be a speck at each place where the lines of force cut 
the front, but only at those places where the lines of force happen 
to be in vibration at the instant under consideration. The energy 
of the wave is thus collected into isolated regions, these regions 
being the portions of the lines of force occupied by the pulses 
or wave motion. In fact, from this point of view the distribution 
of energy is very like that contemplated on the old emission 
theory, according to which the energy was located on moving 
particles sparsely disseminated throughout space. The energy is 
as it were done up into bundles and the energy in any particular 
bundle does not change as the bundle travels along the line of 
force. Thus, if we consider light falling on a metal plate, if we 
increase the distance of the source of light we shall diminish the 
number of these different bundles or units falling on a given area 
of the metal, but we shall not diminish the energy in the indi- 
vidual units; thus any effect which can be produced by a unit by 
itself, will, when the source of light is removed to a greater 
distance, take place less frequently it is true, but when it does 
take place it will be of the same character as when the intensity 
of the light was stronger. This is I think the explanation of the 
remarkable result discovered by Lenard, that though the number 
of corpuscles emitted by a piece of metal exposed to ultra-violet 
light increases as the intensity of the light increases, the velocity 
with which individual corpuscles come from the metal does not 
depend upon the intensity of the light. If this result stood alone 
we might suppose that it indicated that the forces which expel 
the corpuscles from the metal are not the electric forces in the 
28 
VOL. XIV. PT. IV. 
