164 BELL SYSTEM TECHXICAL JOURNAL 



electrons, ninety-two, tlian an\- other element). The complexity of 

 the spectra results from tiiis richness of electrons, but the electrons 

 extracted from the L and M levels of gold In- its own radiations can 

 be identified. 



It is not necessary to proxide an X-ray tuht- ti> su[)pl\' tlu' primary 

 radiation; this can be supplied from the nuclei of radioactive atoms 

 mingled with the atoms being tested, or, by examining radioacti\-e 

 substances, we can discover electronic spectra excited by radiations 

 originating at the nuclei of the atoms themselves. Actually these 

 were the earliest electronic spectra discowred ; the first to be observed 

 were photographed by von Baeycr, Hahn, and Meilner in 1910, years 

 before the interpretation was made (the frequencies of the nuclear 

 radiations were not then known). The figures 1, 2, 3 and 4, used to 

 illustrate this article, arc taken from a paper by J. Danysz, describing 

 work performed in 1911 at the laboratory of Madame Curie in Paris, 

 upon the electrons or bcta-ra\s emerging from atoms of radium B and 

 radium C. The grouping of these electrons, as we now know, results 

 from their being extracted from the various le\els by the several 

 nuclear radiations and the inevitable secondary radiations which 

 they produce in their own atoms. The large number of distinct 

 groups (Rutherford and Robinson distinguished sixteen from radium 

 B and forty-eight from radium C) is \ery likely due to several co- 

 operating causes; there are several frequencies at work, the atoms 

 have large numbers of electrons, and extractions probably occur 

 exceptionally often where the radiations originate so close to ilie 

 electrons. The earliest electronic spectra produced from non-radio- 

 active atoms were excited by nuclear rays from radioactive sub- 

 stances, and the earliest rule discovered was that these spectra were 

 very similar to the spectra of the radioactive atoms themselves; being 

 indeed identical when the excited atoms are isotopes of the atoms 

 which emit tlie exciting rays. In a complete account of this topic, 

 many other names would be mentioned, notably those of (". D. llllis 

 and R. Whiddington. 



A recently-published and relatively simjile case is that of the 

 radioactive atom, uranium A'l, of which the electronic spectrum is 

 shown in Fig. G (from an article by Frl. Meitner). This displays 

 three lines made by electrons of which the speeds indicate that they 

 are extracted from the L, M and A^ levels of the atom by a single 

 radiation, having itself the frequenc>- of the natural A'a-radiation of 

 the atom. This radiation was itself delected and identified l)\- ap- 

 propriate means. Faster electrons which were also observed, cannot 

 have been derixed from an\- such source; tiie\' proh.ibK- came frnni 



