SECTIONAL ADDRESSES. 
ON THE ORIGIN OF SPECTRA 
(RECENT PROGRESS). 
ADDRESS TO SECTION A (MATHEMATICS AND PHYSICS) BY 
Proressor J. C. McLENNAN, F.R.S., 
PRESIDENT OF THE SKCTION. 
Introduction. 
Tue problem of the origin of spectra is intimately bound up with that 
of the constitution and structure of atoms. Models of atoms of different 
types have been proposed from time to time, and these all have served, 
in a measure, to explain some at least of the chemical, optical, and 
mechanical properties of matter. The conception, however, that in- 
spires and co-ordinates the whole of modern atomic physics in so far as 
radiation is concerned is the remarkably simple atomic model of Ruther- 
ford and Bohr. 
According to this model the neutral atom consists of a central 
positively charged nucleus with dimensions of the same order as those 
of the electron itself (10-** cm.),’ and surrounded by a system of elec- 
trons whose aggregate negative charge is equal in amount to that of the 
positive charge carried by the nucleus. The atomic nwmber—t.e. the 
number that indicates the places occupied by the element under con- 
sideration in the Periodic 'Table—gives for a neutral atom the number 
of electrons surrounding the nucleus, and is at the same time a measure 
of the positive electric charge ¢darried by the latter. 
Rutherford, by his brilliant experiments on the scattering of alpha 
rays, has shown that the electric field due to the charge on the nucleus 
is central, and that it follows the inverse square law practically up 
to the effective boundary of the nucleus. Close to the nucleus the 
electric field is very intense, and therefore sufficient to produce those 
remarkably interesting deflections of alpha rays that are being studied 
so widely and so successfully at the present time by the use of 
C. T. R. Wilson’s beautiful method of photographing cloud tracks. 
As regards the problem of the origin of spectra, but little progress 
was made so long as one limited oneself to the use of classical mechanics. 
With the introduction of the theory of quanta into the mechanics of 
the atom it became possible to analyse in detail the structure of atoms 
and to make quantitative comparisons between the properties of matter 
and those deducible from the different atomic models. In the develop- 
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ments that have taken place in this direction Niels Bohr has been the 
leader ; but very notable and important contributions to the theory have 
1 Neuberger, Ann. der Phys., Bd. 70, Heft 2, p. 139, 1923. 
