ce of even
the personality of the observer. This standard is no longer, as
formerly, a flat rule, weak and fragile, but a rigid bar, incapable of
deformation, in which the material is utilised in the best conditions
of resistance. For a standard with ends has been substituted a
standard with marks, which permits much more precise definition and
can be employed in optical processes of observation alone; that is, in
processes which can produce in it no deformation and no alteration.
Moreover, the marks are traced on the plane of the neutral fibres[2]
exposed, and the invariability of their distance apart is thus
assured, even when a change is made in the way the rule is supported.

[Footnote 2: The author seems to refer to the fact that in the
standard metre, the measurement is taken from the central one of three
marks at each end of the bar. The transverse section of the bar is an
X, and the reading is made by a microscope.--ED.]

Thanks to studies thus systematically pursued, we have succeeded in
the course of a hundred years in increasing the precision of measures
in the proportion of a thousand to one, and we may ask ourselves
whether such an increase will continue in the future. 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.

This was determined in 1894 by M. Michelson and M. Benoit in