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BELL SYSTEM TECHNICAL JOURNAL 



plate at C. The rays are excited at the centre of a tube 1' communicat- 

 ing by the slit with the grating-chamber. In this instance the source of 

 light was a vacuum-spark between the electrodes sketched ; had it been 

 a vacuum arc or a glow-discharge in a permanent gas, the tube might 

 have been difTerent in appearance, but would have been sealed onto 



Fig. 2 — Vacuum spectrograph with concave grating. {Proceedings of the Royal Society.) 



the chamber at the slit in the same manner. The distance SL and 

 LC are each one metre, and the sum of them constitutes the major 

 part of the light-path (Lyman has reduced the sum to 40 cm. by 

 using a more curved grating). 



The extension of the explored or explorable region of the spectrum 

 from 1200A onward to 136A does not entirely close the lacuna; but 

 it brings into the accessible range every one of a certain very im- 

 portant class of rays — the rays emitted by a free atom when its 

 \'alence-electron has been displaced and is returning towards or to 

 its normal position. The reason for distinguishing one electron of 

 the atomic electron-system above the others as the valence electron 

 (the name is chosen rather for its meaninglessness than for its mean- 

 ing) lies in the existence of line-series in the spectra. Magnificently 

 regular series of rays are observed in the spectra of the atoms of 

 hydrogen and of ionized helium, each of which has an electron-system 

 consisting of a single electron in the inverse-square field surrounding 

 the atom-nucleus.' Series which resemble these, though tiicy are not 

 arranged according to so elegantly simple a numerical law, are found 

 in the spectra of the elements of the first column of the periodic table 

 (Fig. 3) and suggest forcibly that one of the electrons of the atom 

 of lithium, or sodium, or potassium lies so much farther out than all 

 the others that it moves by itself in a field which is almost identical 

 with the inverse-square field of a nucleus of charge e (the resultant 

 of the fields of the nucleus and the inner electrons approaches such a 



' This inverse-square field seems to be assured by the experiments on deflections 

 of alj)ha and beta particles by atom-nuclei, quite apart from the successes of Bohr's 

 special assumptions about atomic structure and radiation. 



