wave-lengths of the red, green, and blue radiations of cadmium, and
then in terms of the standard metre. The great difficulty of the
experiment proceeds from the vast difference which exists between the
lengths to be compared, the wave-lengths barely amounting to half a
micron;[3] the process employed consisted in noting, instead of this
length, a length easily made about a thousand times greater, namely,
the distance between the fringes of interference.

[Footnote 3: I.e. 1/2000 of a millimetre.--ED.]

In all measurement, that is to say in every determination of the
relation of a magnitude to the unit, there has to be determined on the
one hand the whole, and on the other the fractional part of this
ratio, and naturally the most delicate determination is generally that
of this fractional part. In optical processes the difficulty is
reversed. The fractional part is easily known, while it is the high
figure of the number representing the whole which becomes a very
serious obstacle. It is this obstacle which MM. Michelson and Benoit
overcame with admirable ingenuity. By making use of a somewhat similar
idea, M. Macé de Lépinay and MM. Perot and Fabry, have lately effected
by optical methods, measurements of the greatest precision, and no
doubt further progress may still be made. A day may perhaps come when
a material standard will be given up, and it may perhaps even be
recognised that such a standard in time changes its length by
molecular strain, and by wear and tear: and it will be further noted
that, in accordance with certain theories which will be noticed later
on, it is not invariable when its orientation is changed.

For the moment, however, the need of any change in the definition of
the unit is in no way felt; we must, on the contrary, hope that the