SIDELIGHTS ON RELATIVITY

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for the separate atoms of the same chemical element would not be
in such exact agreement as experience demonstrates. The existence
of sharp spectral lines is a convincing experimental proof of the
above-mentioned principle of practical geometry. This is the ultimate
foundation in fact which enables us to speak with meaning of the
mensuration, in Riemann's sense of the word, of the four-dimensional
continuum of space-time.

The question whether the structure of this continuum is Euclidean,
or in accordance with Riemann's general scheme, or otherwise,
is, according to the view which is here being advocated, properly
speaking a physical question which must be answered by experience,
and not a question of a mere convention to be selected on practical
grounds. Riemann's geometry will be the right thing if the laws
of disposition of practically-rigid bodies are transformable into
those of the bodies of Euclid's geometry with an exactitude which
increases in proportion as the dimensions of the part of space-time