PRELIMINARY SURVEY 
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mass of an electron, and h is Planck’s constant. Taking the 
most recent determinations * of m, e and h, we find 
r e© — 3*286 x io 15 seer 1 . 
Spectroscopic measurements of wave-lengths and a know 
ledge of the speed of light lead to the experimental value of 
Rydberg’s constant, v x = 3-2906 x io 15 sec. -1 . 
But the matter does not end here. The results require 
slight modification in consequence of the fact that as the mass 
of the nucleus is not infinite, the nucleus itself must describe 
an orbit. When this is taken into account, it is possible 
to form an estimate of the ratio between the mass of the 
electron and the mass of the nucleus from spectroscopic obser 
vations of the lines of hydrogen and helium. 
The discovery by C. G. Barkla of “ fluorescent ” X-radiation 
characteristic of a particular element and the distinction drawn 
by him between the harder “ K ” rays and the softer “ L ” rays 
prepared the way for a complete study of the spectrum of X-rays. 
Rapid progress in this branch of physics was made after the 
introduction of the X-ray spectrometer in which a crystal serves 
as a diffraction grating. In Bohr’s theory the penetrating K 
radiation is attributed to the fall of an electron to the lowest 
atomic “ level ”—or the innermost “ electron ring ”—the “ L ” 
radiation being due to the fall of an electron to the “ level ” 
above this, corresponding to the next most stable position for 
an electron. 
The application of the quantum theory to X-ray spectra has 
yielded results of great interest in connection with the structure 
of the atom, but the subject is too wide to enter upon here and 
the discussion of it may profitably be deferred. 
5. The Generalized Quantum Theory 
Further progress has been made possible by a later and 
more general formulation of the quantum theory put forward 
by W. Wilson, Sommerfeld, Ishiwara and several others, which 
has linked together the various interpretations given for the 
quantum constant. It is supposed that each physical system 
behaves normally in a conservative way, its motion being subject 
to Hamiltonian dynamics. It is then said to be in one of its 
stationary states. Interchanges of energy between physical 
systems are the result of catastrophes or revolutions in the atom. 
These interchanges are discontinuous in character and so involve 
the emission or absorption of an amount of energy depending 
on the initial and final state. The characteristic and funda- 
*See Appendix II, p. 257.