He early directed his researches to the velocity of light and while in Cleveland invented his interferometer (see 14.693), which enabled him to measure distances by means of the length of light-waves.
Magnusson, is to introduce a thin film of the dye into one of the optical paths of a Michelson interferometer, and to determine the consequent displacement of the fringes.
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) .
The echelon and interferometer serve only a limited purpose, but must be called into action when the detailed structure of lines is to be examined.
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).
Michelson, who has compared the theoretical widening with that found experimentally by means of his interferometer, had to use a somewhat more complicated expression for the comparison, as his visibility curve does not directly give intensities for particular frequencies but an integral depending on a range of frequency?