PROGRESS IN PHYSICS—THOMSON. 199 
We may picture to ourselves the radiation as consisting of the lines 
of electric force which, before the vibrations were started, were held, 
bound by the charges on the jar, and which, when the vibrations be- 
gin, 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 frac- 
tions 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 ght 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and withdrawn as a whole from the pencil of light incident on the 
condenser. 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 con- 
denser 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 illustration in the remarkable laws governing sec- 
ondary Rontgen radiation, recently discovered by Professor Barkla. 
Professor Barkla has found that each of the different chemical ele- 
ments, when exposed to Réntgen rays, emit a definite type of second- 
ary 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 radia- 
tion from lead being harder. than that from copper; if copper is ex- 
posed to the secondary radiation from lead the copper will radiate, 
but lead will not radiate when exposed to copper. Thus, if we sup- 
pose 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 ulti- 
mately be many thousands of miles apart, it would seem meaningless 
45745°—sm 1909——14 
