No doubt
progress will not be stayed; but if we keep to the definition of
length by a material standard, it would seem that its precision cannot
be considerably increased. We have nearly reached the limit imposed by
the necessity of making strokes of such a thickness as to be
observable under the microscope.
It may happen, however, that we shall be brought one of these days to
a new conception of the measure of length, and that very different
processes of determination will be thought of. If we took as unit, for
instance, the distance covered by a given radiation during a
vibration, the optical processes would at once admit of much greater
precision.
Thus Fizeau, the first to have this idea, says: "A ray of light, with
its series of undulations of extreme tenuity but perfect regularity,
may be considered as a micrometer of the greatest perfection, and
particularly suitable for determining length." But in the present
state of things, since the legal and customary definition of the unit
remains a material standard, it is not enough to measure length in
terms of wave-lengths, and we must also know the value of these
wave-lengths in terms of the standard prototype of the metre.
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