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.