The now numerous public which tries with some success to keep abreast
of the movement in science, from seeing its mental habits every day
upset, and from occasionally witnessing unexpected discoveries that
produce a more lively sensation from their reaction on social life, is
led to suppose that we live in a really exceptional epoch, scored by
profound crises and illustrated by extraordinary discoveries, whose
singularity surpasses everything known in the past. Thus we often hear
it said that physics, in particular, has of late years undergone a
veritable revolution; that all its principles have been made new, that
all the edifices constructed by our fathers have been overthrown, and
that on the field thus cleared has sprung up the most abundant harvest
that has ever enriched the domain of science.

It is in fact true that the crop becomes richer and more fruitful,
thanks to the development of our laboratories, and that the quantity
of seekers has considerably increased in all countries, while their
quality has not diminished. We should be sustaining an absolute
paradox, and at the same time committing a crying injustice, were we
to contest the high importance of recent progress, and to seek to
diminish the glory of contemporary physicists. Yet it may be as well
not to give way to exaggerations, however pardonable, and to guard
against facile illusions. On closer examination it will be seen that
our predecessors might at several periods in history have conceived,
as legitimately as ourselves, similar sentiments of scientific pride,
and have felt that the world was about to appear to them transformed
and under an aspect until then absolutely unknown.

Let us take an example which is salient enough; for, however arbitrary
the conventional division of time may appear to a physicist's eyes, it
is natural, when instituting a comparison between two epochs, to