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PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 
from right to left, i.e., from long to short waves, we see that sudden increases in 
absorption occur when the primary radiation becomes of shorter wave-length than the 
radiations characteristic of the absorbing substance—that is, when the characteristic 
radiations begin to be excited in the absorbing substance—and that the absorption 
afterwards gradually approaches proportionality with the absorption in the standard 
substance—as shown by the approximation to a straight line through the origin. 
Thus the line ABB' shows what would be the absorption of radiations of various 
wave-lengths if there were no L characteristic radiation excited in the absorbing 
substance. The rise as shown by the portion BCD is associated with the emission by 
the absorbing substance of the fluorescent X-radiation of series L. The difference 
!K 
Absorbability in standard substance 
or (wave-length) 3 —approximately. 
Fig. 2. 
between the ordinates of the curve BCD and BB' gives the absorption associated 
with the emission of L radiation, or what we shall call the “ L ” absorption (E L ). 
In a similar fashion we can get the “ K ” absorption E K . The lower portion of fig. 2 
indicates the various independent absorptions S, E K , E L , E M N ... in separate curves 
where S = energy absorbed and re-emitted as scattered radiation, E K , E L , E Mi N ... = 
energy absorbed in association with the emission of characteristic radiations of series 
K, L, and M, N, ... respectively. The absorption S due to the process of scattering 
is approximately independent of wave-length, so is represented by a horizontal line. 
It however rises considerably at the long wave-length end if the wave-length be 
sufficiently great. 
