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PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
amount of polarized radiation which ought very easily to have been observed in 
Beatty’s experiments if it differed in method of production from that of the bulk 
of the heterogeneous primary radiation.) 
If in the phenomenon of X-ray fluorescence we find it is essential that an electron 
be displaced from its normal position, and are led to conclude that X-rays are 
produced when the displaced or some other electron falls back into this position, we 
ought to look for a similar process in explanation of the more direct generation 
of X-rays. This naturally leads to the following considerations :— 
The cathode particles in the cathode stream by collision expel electrons from their 
normal positions in the atoms in the anti-cathode ; these displaced electrons thus receive 
an amount of energy—measured from the normal position and state—which must be 
less than the original kinetic energy of the impinging electron and may be much less. 
When they fall back to their normal positions they re-emit as radiation the potential 
energy which they lose in the fall. Further, if an electron in falling from one 
definite position to another emits radiation whose frequency n is given by the relation 
hn — energy lost in falling from position to position—a law for which we have given 
direct experimental evidence and which was assumed by Bohr —we need only assume 
an extension of the law to apply to any fall within the atom. We must assume that 
when an electron falls from any position within an atom to some normally stable 
position it emits radiation, a quantum of which is equal to the potential energy lost 
in the fall. 
Thus when an electron falls back from its position of greatest displacement, the 
frequency of the resultant radiation is given by hn — potential energy of electron in 
its position of greatest displacement measured from its normal position and state — 
which may have any value less than the original kinetic energy of the cathode particle. 
When it happens that the place of the displaced electron is taken by an electron from 
one of the outer rings in the atom, a characteristic radiation is emitted. 
Such a mixture of heterogeneous and characteristic radiations is actually emitted 
from the anti-cathode. This theory suggest a definite upper limit to the penetrating 
power or frequency of the radiation possibly emitted by an anti-cathode, for the 
highest frequency would be that generated when an electron falls freely from the 
surface of the atom to the innermost stable position of an electron, on the assumption 
that such a limit exists. A limit to the frequency of the X-radiation generated in 
a particular anti-cathode has not yet been proved, nor is there any certain experimental 
evidence in support of the hypothesis, though from considerations indicated above 
it seems probable theoretically. 
[Though results have been obtained by certain experimenters which from a superficial 
examination appear to indicate such a limit, these results might have been foretold 
not only qualitatively, but with fair accuracy quantitatively, from the observed laws 
of scattering and absorption. 
An approximate limit to the penetrating power of a radiation does not indicate a 
