34 SECTIONAL ADDRESSES, 
perfectly definite quantity. No doubt it is when the condition of the 
body is or can be definitely specified, but it is extraordinarily sensitive 
to minute changes in the conditions of the surface, such as may be 
caused, for example, by the presence of extremely attenuated films of 
foreign matter. or this reason we should accept with a certain degree 
of reserve statements which appear from time to time that photoelectric 
action is some parasitic phenomenon, inasmuch as it can be made to 
disappear by improvement of vacuum or other change in the conditions. 
What has generally happened in these investigations is that something 
has been done to the illuminated surface which has raised its threshold 
frequency above that of the shortest wave-length in the light employed 
in the test. Unless they are accompanied by specific information 
about the changes which have taken place in the threshold frequency, 
such statements are of little value at the present stage of development 
of this subject. 
Interesting calculations have been made by Frenkel which bring 
surface tension into close connection with the thermionic work w. 
Broadly speaking, there can be little doubt that a connection of this 
nature exists, but whether the relation is as simple as that given by 
the calculations is open to doubt. It should be possible to answer this 
question definitely when we have more precise information about the 
disposition of the electrons in atoms such as the continuous progress 
in X-ray investigation seems to promise. 
Light and X-Rays. 
One of the great achievements of experimental physics in recent 
years has been the demonstration of the essential unity of X-rays and 
ordinary light. X-rays have been shown to be merely light of particu- 
larly high frequency or short wave-length, the distinction between the 
two being one of degree rather than of kind. The foundations of our 
knowledge of X-ray phenomena were laid by Barkla, but the discovery 
and development of the crystal diffraction methods by v. Laue, the 
Braggs, Moseley, Duane, and de Broglie haye established their relations 
with ordinary light so clearly that he who runs may read their substan- 
tial identity. The actual gap in the spectrum of the known radiations 
between light and X-rays is also rapidly disappearing. The longest 
stride into the region beyond the ultra-violej was made by Lyman 
with the vacuum grating spectroscope which he developed. For a short 
time Professor Bazzoni and I held the record in this direction with 
our determination of the short wave limit of the helium spectrum, 
which is in the neighbourhood of 450 Angstrom units. More recently 
this has been passed by Millikan, who has mapped a number of lines 
extending to about 200 Angstrom units—that is to say, more than four 
octaves above the violet limit of the visible spectrum. I am not sure 
what is the longest X-ray which has been measured, but I find a 
record of a Zine L-ray by Friman® of a wave-length of 12.346 
Angstrom units. There is thus at most a matter of about four octaves 
° Phil. Mag., vol. xxxii., p. 494 (1916). 
