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THE QUANTUM [xvm. 4 
XII), and as a matter of fact Rutherford and Chadwick have 
themselves shown that this law fails when the distance is 
sufficiently diminished.* After the anticipatory work of Nichol 
son came the bold advance made by Bohr in introducing quantum 
conditions for the determination of stationary states, and a 
new hypothesis for determining the amount of radiation emitted 
or absorbed in a quantum transition. 
Although there has been considerable discussion in the past 
as to the “ correctness ” of Bohr’s atomic model, such a discussion 
is almost certainly entirely superfluous at the present time when 
Bohr himself, and many other physicists with him, would probably 
say that such a model has, for the time being, a heuristic value 
and serves the purpose of a good working hypothesis. There is, 
indeed, a tendency in some quarters to go to extremes and 
discard all attempts at constructing models of the ultimate 
physical entities. Let us express in mathematical language, 
say some philosophers, only these results which are capable of 
actual observation, and put on one side all such mechanistic 
interpretations as require, for instance, a knowledge of the 
position in space at a particular instant of time of a particular 
electron. 
The conflict between the rival theories of light which has 
already been described has its counterpart in the two views which 
may be taken as to the constitution of matter. These might be 
somewhat inadequately described as the static and the dynamic 
views. The wave-theory of matter associated with the names 
of de Broglie and Schrödinger emphasizes the idea that a mass 
particle is associated with or is equivalent to a condition of 
vibration. It may be that the solution of our difficulties in 
regard to the rival theories, both of radiation and of matter, 
will eventually be found in a more general theory which will 
include both radiation and matter in an extended wave 
mechanics. 
4. The New Quantum Mechanics 
The new quantum mechanics affords an instructive example 
of two contrasted methods of attacking a physical problem. The 
matrix mechanics is based on the principle that only such 
quantities as are directly open to observation are to be employed 
in the mathematical formulation ; the undulatory mechanics 
is based on the assumption that an atomic system, composed of 
electrified particles, may be treated as equivalent to a wave- 
motion determined by a special form of wave-equation. Although 
* See the Twelfth Guthrie Lecture, “ Atomic Nuclei and their Trans 
formations,” Rutherford, Pvoc. Phys. Soc., vol. 39, p. 359, 1927.