200 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
with which we have been dealing have frequencies which make them 
the equivalent of from 12 to 30 million volt rays. It is not improb- 
able that the capture of an electron by the nucleus of a ight atom 
involves a higher energy than its capture by a heavy one, so that 
such captures as are here discussed constitute, perhaps, the most 
plausible hypothesis as to the origin of these rays. 
Is it possible to imagine such a phenomenon going on all through 
space? The difficulty is not so insuperable, in view of the trans- 
parency even of large amounts of matter for these hard rays combined 
with Hubbell’s recent proof +* at the Mount Wilson Observatory that 
some of the spiral nebulae are at least 1,000,000 light-years away. 
The centers at which these nuclear changes are taking place would 
then only have to occur at extraordinarily widely scattered intervals 
to produce the intensity of the radiation observed at Muir Lake. 
The only alternative hypothesis to that above presented of high- 
frequency rays transvering space in all directions, might seem to be 
to assume that the observed rays are generated in the upper layers 
of the atmosphere by electrons shooting through space in all direc- 
tions with practically the speed of light. This hypothesis might 
help some in interpreting the mysterious fact of the maintenance of 
the earth’s negative charge, but it meets with insuperable obstacles, 
I think, in explaining quantitatively the variation with altitude of 
the ionization in closed vessels. In any case, in its most important 
aspect, this hypothesis is very much like the one presented above, 
for it, too, fills space with rays of one sort or another traveling in 
all directions with the speed of light. From some such conception 
as this there now seems to be no escape. And yet it is a conception 
which is almost too powerful a stimulus to the imagination. Pro- 
fessor MacMillan of Chicago will wish to see in it evidence for the 
condensation into matter out somewhere in space of the light and 
heat continually being radiated into space by the sun and stars,’* an 
altogether permissible speculation. Unfortunately the psychics will 
of course be explaining all kinds of telepathies with the aid of these 
cosmic rays. But, be that as it may, the simple experimental facts, 
as shown by the foregoing work, are: 
(1) That these extraordinary penetrating rays exist; 
(2) That their mass absorption coefficient may be as high as 0.18 
per meter of water; 
(3) That they are not homogeneous, but are distributed through 
a spectral region far up above X-ray frequencies—probably one 
thousand times the mean frequencies of X rays; 
18 Hubbell, Pop. Astron., 33, pp. 252—255, 1925. 
44 MacMillan, Science, 62, 122, 1925. 
