PROF. C. G. BARKLA ON X-RAYS AND THE THEORY OF RADIATION. 339 
Each Fluorescent X- radiation Associated with Particular Group of Electrons. 
But of the electrons emitted with a velocity greater than the critical velocity, 
that is, with energy greater than that of a quantum of the fluorescent X-radiation, 
only a fraction are accompanied by the particular fluorescent radiation, say of 
series K. # 
As pointed out, the total corpuscular radiation, though consisting of electrons ejected 
with one velocity, may be divided into several distinct portions, each portion being 
definitely associated with the emission of fluorescent radiation of a definite series. Thus 
the whole corpuscular radiation consists of Kt electrons, associated with the emission 
of the characteristic X-radiation of series K, L electrons associated with the 
characteristic X-radiation of series L, and M, N, &c., electrons presumably associated 
with the emission of hypothetical lower frequency characteristic X-radiations of series 
M, N, &c. When few K electrons are emitted, the K fluorescent X-radiation is 
very weak ; when K electrons increase in number the intensity of the fluorescent 
X-radiation (series K) increases, and indeed, as we shall see, there is certainly approxi¬ 
mate if not perfectly accurate proportionality between the number of K electrons 
ejected and the intensity of the fluorescent X-radiation of series K. On the other 
hand, there is no simple relation between the intensity of fluorescent radiation of a 
particular series and the total number or energy of the electrons in the whole corpus¬ 
cular radiation. 
•Connection with Absorption of Primary Radiation and with Corpuscular Radiation 
{Radiation in Quanta). 
It has been seen that of the energy definitely absorbed in association with the 
emission of fluorescent X-radiation of series K—-K absorption—the fraction re¬ 
emitted as fluorescent radiation (series K) is at its maximum value when the primary 
radiation is of just shorter wave-length than the fluorescent radiation itself, and that 
this fraction decreases with a decrease in wave-length of the primary radiation. Thus 
if F k is the energy of the fluorescent radiation (series K) u and // K are the wave¬ 
lengths of the primary and fluorescent radiations respectively, then as u decreases 
from /u K downward F K /E K decreases from about half, as shown in fig. 5. 
On the other hand, as p. decreases from /x K downward, of the energy E K specially 
absorbed in connection with the emission of K radiation, an increasing fraction re-appears 
as energy of corpuscular radiation (C K ). In the case of the one substance examined 
in detail, and approximately in that of other substances, C K /E K increases from about 
^ towards 1. Thus when /u is just less than /a K the energy of the primary radiation 
specially absorbed is approximately equally divided between the two types of secondary 
* See Barkla, “ Charakteristische Rontgenstrahlungen ” (‘ Verb. d. Deut. Phys. Gesell.,’ 1913). 
1 - f The case here considered is that in which the “ J ” radiation (which from most elements is of very 
high frequency) is not excited. 
3 B 
VOL. CCXVTI.—A. 
