PRESENT CONCEPTION OF AN ATOM MILLIKAN 191 



constitute structures which are all exactly alike, save that the fields 

 in which the single electron which is left is describing its orbit 

 increase in the ratios one, two, three, four, five, as we go from 

 stripped lithium to stripped nitrogen. Now we have applied the 

 relativity doublet formula which, as indicated above, Sommerfeld 

 had developed for the simple nucleus-electron system found in 

 hydrogen and ionized helium, and have found that it not only 

 predicts everywhere the observed doublet separation of the spectra 

 produced by all these stripped atoms, but that it enables us to com- 

 pute the effect which the two electrons close to the nucleus of all 

 these atoms have in screening the outer rotating electron from 

 this nucleus. 



At a suiRcient distance from the nucleus these two electrons ought 

 to neutralize exactly two of the free positive charges on the nucleus, 

 provided, and only provided, the forces emanating from these elec- 

 trons fall off with the inverse square of the distance. Our relativity 

 doublet formula, with this assumption and without the introduction 

 of any arbitrary constants whatsoever, enabled us to predict what 

 the screening effect due to those two electrons ought to be. And now 

 our experiments on doublet separations reveal that that screening is 

 within one per cent of two, which checks with what we knew before- 

 hand, from radioactive and chemical data, that it ought to be. In 

 other words, we have another method which enables us with certainty 

 to look inside the atom and find how many electrons are in the inmost 

 shell, and the answer comes out two. 



Again, when we examine the spectrum due to the stripped atoms 

 of the group of atoms from sodium to chlorine — one electron having 

 been knocked off from sodium, two from magnesium, three from 

 aluminium, four from silicon, five from phosphorus, six from 

 sulphur, and seven from chlorine — we should find in every case that 

 the number of screening electrons in the two inmost shells, when 

 tested for sufficiently remote orbits, comes out two plus eight, i. e., 

 ten. And it does come out in every case very nearly as predicted. 

 This constitutes excellent evidence that the electrons at these dis- 

 tances and under these conditions do possess Coulomb fields (fields 

 falling off with the inverse square of the distance), a result appar- 

 ently incompatible with the loafer-electron theory. The physicist 

 has thus piled Ossa on Pelion in his quantitative proof of the exist- 

 ence of these electron orbits. 



These new results are, however, incompatible with the precise 

 shapes of orbits with which the physicists have been working 

 in the field of optics during the last five years. They necessitate 

 either the abandonment of the relativity cause for the separation 

 of our measured spectroscopic doublets or else they require us to 



