40
DOC.
5
CONTRIBUTIONS
TO
QUANTUM THEORY
[20]This
point
was
discussed
in
more
detail
in Ehrenfest 1914.
The connection with Wien's
displacement
law
was
already emphasized by
Ehrenfest
in
this
paper,
as
well
as
in
Ehrenfest
1913.
[21]See
also
the related
discussion in Doc. 26.