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58 plane-mirror being fixed to the arm. Moreover, one measures, not the distance between two consecutive fringes, but that between two symmetrical fringes of a high order—for example, that between the tenth fringe on the right, and the tenth fringe on the left. From these measures of angle, and from the number of lines per millimetre of the grating, the wave-length can be deduced by the known formula.

Each wave-length has been determined by three series of measures, effected with three gratings, having respectively 200, 100, and 50 lines per millimetre.

The following table exhibits the results of these measures:—

Being desirous of controlling these determinations by the use of a quite different method,