By making use of a somewhat similar idea, M.Mace de Lepinay 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 use of the unit adopted by the physicists of the whole world will spread more and more.

It is right to remark that a few errors still occur with regard to this unit, and that these errors have been facilitated by incoherent legislation.

France herself, though she was the admirable initiator of the metrical system, has for too long allowed a very regrettable confusion to exist; and it cannot be noted without a certain sadness that it was not until the _11th July 1903_ that a law was promulgated re-establishing the agreement between the legal and the scientific definition of the metre.
Perhaps it may not be useless to briefly indicate here the reasons of the disagreement which had taken place.