PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
321 
result of the passage of the primary beam, but arises only indirectly from it; the 
process is dependent on some critical condition, as evidenced by Stokes’s law. Here 
at once we see the possibility of the applicability of some kind of quantum theory. 
The most significant evidence as to the origin of the characteristic radiation comes 
from a study of the accompanying phenomena of (l) absorption of the exciting 
primary radiation ; and (2) emission of electrons by the radiating substance, in 
the form of a corpuscular radiation. It is necessary here to introduce a brief account 
of these phenomena. 
Absorption .—The absorption of a primary radiation is due to various causes which 
can be quite readily distinguished ;* they are distinct and apparently independent 
absorptions. Thus the total absorption in a particular element of radiations of various 
wave-lengths may be represented by a curve of the form shown in fig. 1, the ordinates 
in which indicate the absorptions of radiations whose wave-lengths are given by the 
abscissae. The curve is of simpler form however, if instead of wave-lengths we plot 
as abscissae the absorbabilities in some standard substance which has no spectral lines 
in the part of the spectrum considered.! Then the absorption curve takes the form of 
fig. 2. This, however, for the sake of clearness is not drawn to scale. The lower 
portion of the figure gives the spectral lines of the substance traversed.^ Proceeding 
* Barkla and Sadler, ‘Phil. Mag.,’May, 1909 ; Barkla and Collier, ‘Phil. Mag.,’ June, 1912; 
Barkla, ‘ Proc. Phil. Soc. Camb.,’ May, 1909 ; Barkla and Sadler, ‘Nature,’ July 16, 1908, and 
March 11, 1909. 
t Or the (wave-length) 3 approximately, as Hull and Rice have shown. 
t These curves do not show the exact relation in the regions of very rapid change, as the radiations 
used were not perfectly homogeneous. 
