18, 4 _ Perkins: The Structure of the Electron 839 
According to this view a hydrogen molecule looks somewhat 
like this (§), and a helium nucleus the same only some- 
thing like a thousand times smaller with two extra positive 
units stably bound because of the high local concentration of 
negative charge. : 
So much for pure deduction from the classical theory 
toward explaining the stability and definiteness of atoms. This 
could have been done twenty years ago * except for the fixed 
idea of a point electron. In fact the discovery of radio-activity 
might have been predicted if the electromagnetic theory had been 
logically applied to the atomic problem in the nineteenth 
century. 
Having found a working hypothesis for the most fundamental 
part of our problem, namely, for the existence of the atom, we 
are in a position to return to the spectral lines of the elements, 
in which there are volumes of information as to the velocities, 
dimensions, magnetic flux, and internal properties of electrons 
in atoms. These volumes are somewhat complicated, and a de- 
tailed consideration of radiation will be deferred to a later paper. 
It seems, however, that each spectral series has a limiting fre- 
quency, which has an effect on all the lines of the series and 
often on one line in each of many other series as well. This leads 
us to infer that each limiting frequency is the frequency of rota- 
tion of an electron, upon which other frequencies may be super- 
posed, perhaps by the formation of a vortex ring tangent to the 
main ring (like a large doughnut on a bicycle tire, if we could 
put it on). 
Whatever the exact explanation of radiation we know from 
Moseley’s ** work that each atom has a K-series of a very fun- 
damental character, the limiting frequency of which ‘varies 
approximately with the square of the atomic number. Only 
one such series is known which is uninfluenced by the pres- 
ence of more than one electron, namely, the Lyman (K) series 
of hydrogen. Bohr has shown,** however, beyond reasonable 
“The writer need hardly point out that if the physicists of the nine- 
teenth century were incapable of such a deduction, he himself was infinitely 
less capable of originally arriving at this result by a process of direct 
reasoning. The direct method of attack was apparent to him only after 
several years of mental permutations and commutations of the ideas 
of a score of physicists, notably the idea of constant angular momentum 
due to Bohr, who in turn derived it from Planck’s quantum theory. 
* Cf. Millikan, R, A., Science 45 (1917) 323. 
“Bohr, N., Nature 92 (1913) 231. 
