12 THE PRESIDENTIAL ADDRESS. 
atom Bohr supposed that the single electron could move in a number 
of stable orbits, controlled by the attractive force of the nucleus, with- 
out losing energy by radiation. The position and character of these 
orbits were defined by certain quantum relations depending on one or 
more whole numbers. It was assumed that radiation was only emitted 
when the electron for some reason was transferred from one stable 
orbit to another of lower energy. In such a case it was supposed that 
a homogeneous radiation was emitted of frequency v determined by the 
quantum relation E=hy where E was the difference of the energy of 
the electron in the two orbits. Some of these possible orbits are 
circular, others elliptical, with the nucleus as a focus, while if the 
change of mass of the electron with velocity is taken into account’ the 
orbits, as Sommerfeld showed, depend on two quantum numbers, and 
are not closed, but consist. of a nearly elliptical orbit slowly rotating 
round the nucleus. In this way it is possible not only to account for 
the series relations between the bright lines of the hydrogen spectrum, 
but also to explain the fine structure of the lines and the very compli- 
cated changes observed when the radiating atoms are exposed in a 
strong magnetic or electric field. Under ordinary conditions the 
electron in the hydrogen atom rotates in a circular orbit close to the 
nucleus, but if the atoms are excited by an electric discharge or other 
suitable method, the electron may be displaced and occupy any one 
of the stable positions specified by the theory. In a radiating gas 
giving the complete hydrogen spectrum there will be present many 
different kinds of hydrogen atoms, in each of which the electron 
describes one of the possible orbits specified by the theory. On this 
view it is seen that the variety of modes of vibration of the hydrogen 
atom is ascribed, not to complexity of the structure of the atom, but 
to the variety of stable orbits which an electron may occupy relative 
to the nucleus. This novel theory of the origin of spectra has been 
developed so as to apply not only to hydrogen but to all the elements, 
and has been instrumental in throwing a flood of light on the relations 
and origin of their spectra, both X-ray and optical. The information 
thus gained has been applied by Bohr to determine the distribution of 
the electrons round the nucleus of any atom. The problem is obviously 
much less complicated for hydrogen than for a heavy atom, where each 
of the large number of electrons present acts on the other, and where 
the orbits described are much more intricate than the orbit of the 
single electron in hydrogen. Notwithstanding the great difficulties of 
such a complicated system of electrons in motion, it has been possible 
to fix the quantum numbers that characterise the motion of each 
electron, and to form at any rate a rough idea of the character of the 
orbit. 
These planetary electrons divide themselves up into groups, according 
