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This was determined in 1894 by M. Michelson and M. Benoit in an
experiment which will remain classic. The two physicists measured a
standard length of about ten centimetres, first in terms of the
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.


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