HIGH FREQUENCY RAYS OF COSMIC ORIGIN? 
By R. A. MILLIKAN 
Norman Bridge Laboratory of Physics, California Institute of Technology 
INTRODUCTION BY C. G. ABBOT 
The paper of Professor Millikan which follows may be compared with the 
work of Réntgen, published in the Smithsonian Report of 1897. Some years 
after his discovery of X rays, it was proved that, like light and Hertzian or 
radio rays, they consist of ether vibrations of the transverse type. X rays, 
however, lie in the range of wave lengths from fifty to five thousand times 
shorter than those which produce the sensation of yellow light in the eye. 
Now comes Professor Millikan with the most definite proof thus far obtained 
of a new type of rays also of the nature, as he thinks, of transversely vibrating 
waves, but whose wave lengths are of the order of two thousand times less 
than those of the shortest wave X rays. 
Rontgen’s X rays could penetrate flesh and thereby became a powerful aid 
in surgery and medicine. They could also penetrate many metals opaque to 
ordinary light. But the X rays are stopped by rather thin sheets of lead, so 
that X-ray photographers are accustomed to protect their sensitive plates by 
lead wrappers. Rdntgen, however, distinguished between different degrees of 
penetration in the rays he was able to produce. He introduced (one does not 
exactly know why) the term “ hard” to designate more penetrating and “ soft” 
to designate less penetrating X rays. After the measurements of X-ray wave 
lengths had been accomplished, some years later, the “hard” rays were found 
to differ in being shorter wave lengths than “ soft” ones. 
It is not surprising then that rays of two thousand times less wave length 
should be very “hard.”* Professor Millikan, indeed, finds that these new rays 
will penetrate the equivalent of 6 feet of lead, the most impenetrable of 
common metals for ordinary X rays. 
Still more noteworthy is the fact that the new rays do not appear to be 
engendered on this earth, but rather to fly about in every direction through 
the universe beyond our atmosphere. It is suggested by -Professor Millikan 
that they arise from the destruction of transmutation of atoms in those fiery 
laboratories, the stars. Such features make the subject of the new rays one 
of extraordinary interest and perhaps of great developmental possibilities. 
Readers will be interested to recognize in these new rays a very great addi- 
tion to the gamut of the spectrum described so well by the late Prof. E. F. 
Nichols in his paper in the Smithsonian Report for 1923. Our friends will 
also take pleasure in the thought that the Smithsonian Institution, by its 
support of the work of Langley in the infra-red, of Shumann in the ultra- 
violet, and by its articles in which the progress of knowledge of the extension 
1 Reprinted by permission from the Proceedings of the National Academy of Sciences, 
vol. 12, No. 1, January, 1926. Read before the Academy Nov. 9, 1925. 
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