Perot and C. Fabry, employing their interferometer methods, have compared the wave-length of the red cadmium line with the standard metre in Paris and found it to be equal to 6438.4696 A, the observations being taken in dry air at 18° C and at a pressure of 76 cms. (g = 980.665).
Perot's number is now definitely adopted to define the Angstrom, and need never be altered, for should at some future time further researches reveal a minute error, it will be only necessary to change slightly the temperature or pressure of the air in which the wave-length is measured.
Jacques Perot thanked the treasurer for her scrupulous work on behalf of all members of the Executive Council.
Some other appliance in which interference with long difference of path is made use of, such as the interferometer of Fabry and Perot, or Lummer's plate (see Interference Of Light) .