PRESIDENT’S ADDRESS. 93 
are thrown into rhythmic undulations, liberated from the jar, and travel 
through space with the velocity of light. 
Now let us suppose that this system strikes against an uncharged 
condenser and gives it a charge of electricity, the charge on the plates 
of the condenser must be at least one unit of electricity, because fractions 
of this charge do not exist, and each unit charge will anchor a unit 
tube of force, which must come from the parcel of radiation falling 
upon it. Thus a tube in the incident light will be anchored by the 
condenser, and the parcel formed by this tube will be anchored and 
withdrawn as a whole from the pencil of light incident on the con- 
denser. If the energy required to charge up the condenser with a 
unit of electricity is greater than the energy in the incident parcel, the 
tube will not be anchored and the light will pass over the condenser and 
escape from it. These principles, that radiation is made up of units, and 
that it requires a unit possessing a definite amount of energy to excite 
radiation in a body on which it falls, perhaps receive their best illustra- 
tion in the remarkable laws governing Secondary Réntgen radiation, 
recently discovered by Professor Barkla. Professor Barkla has found 
that each of the different chemical elements, when exposed to Réntgen 
rays, emit a definite type of secondary radiation whatever may have 
been the type of primary: thus lead emits one type, copper another, 
and so on; but these radiations are not excited at all if the primary 
radiation is of a softer type than the specific radiation emitted by the 
substance; thus the secondary radiation from lead being harder than 
that from copper, if copper is exposed to the secondary radiation from 
lead the copper will radiate, but lead will not radiate when exposed to 
copper. ‘Thus, if we suppose that the energy in a unit of hard Réntgen 
rays is greater than that in one of soft, Barkla’s results are strikingly 
analogous to those which would follow on the unit theory of light. 
Though we have, I think, strong reasons for thinking that the energy 
in the light waves of definite wave length is done up into bundles, and 
that these bundles, when emitted, all possess the same amount of energy, 
I do not think there is any reason for supposing that in any casual 
specimen of light of this wave length, which may subsequent to its 
emission have been many times refracted or reflected, the bundles possess 
any definite amount of energy. For consider what must happen when 
a bundle is incident on a surface such as glass, when part of it is 
reflected and part transmitted. The bundle is divided into two portions, 
in each of which the energy is less than the incident bundle, and since 
these portions diverge and may ultimately be many thousands of miles 
apart, it would seem meaningless still to regard them as forming one 
unit. Thus the energy in the bundles of light, after they have suffered 
partial reflection, will not be the same as in the bundles when they 
were emitted. The study of the dimensions of these bundles, for 
example, the angle they subtend at the luminous source, is an interesting 
