XVIII. I] INTERPRETATION OF THE QUANTUM 245 
The marvellous method, due to C. T. R. Wilson, of rendering 
visible and of photographing the track of a single atomic pro 
jectile or a single electron; the scintillations due to alpha 
particles observed in the spinthariscope of Crookes ; the beautiful 
experiments of Rutherford and Geiger in which these particles 
are counted by an ionization method; all these afford convincing 
evidence for the existence of discrete entities which may be called 
atoms of matter or of electricity. 
More recent photographs taken by Wilson’s method, in 
particular those showing the ionization produced by a beam of 
X-rays, might almost be said to reveal the existence of quanta, or 
at least to afford such striking evidence in their favour that even 
the most sceptical must find it hard to disbelieve. 
The controversy between the Realist and the Idealist schools 
of philosophy which has long been raging is reflected in the 
antagonistic views of scientific workers. British physicists, 
inspired by the genius of Newton and influenced by many later 
teachers, have long sought to develop a mechanical theory of 
natural phenomena. They have been accustomed to work by 
means of mechanical models, and as is well known both Faraday 
and Maxwell made extensive use of such models. 
Opposed to this procedure is what may be called the 
Continental method, which does not attach great importance 
to models, but prefers to work after the manner of the pure 
mathematician by means of abstract symbols. Instead of 
dealing with “ action through a medium,” investigators of this 
school recognize “ action at a distance ” and develop the 
consequences of that view. 
To quote again from Schuster : “ Those who believe in the 
possibility of a mechanical conception of the universe and are 
not willing to abandon the methods which from the time of 
Galileo and Newton have uniformly and exclusively led to 
success, must look with the gravest concern on a growing school 
of scientific thought which rests content with equations correctly 
representing numerical relationships between different phenomena, 
even though no precise meaning can be attached to the symbols 
used. The fact that this evasive school of philosophy has 
received some countenance from the writings of Heinrich Hertz 
renders it all the more necessary that it should be treated seriously 
and resisted strenuously.” 
On the other hand we have such a statement as “Maxwell’s 
theory is contained in Maxwell’s equations,” and we meet those 
who regard the idea of a real kinetic theory of matter—at one 
time firmly believed in—as a “ mirage.” Influenced by the 
philosophy of “ positivism ” there is “ a school of physics that 
takes no interest in a mechanism explaining the phenomena, but