A Grating Interferometer

Discussions of the Michelson interferometer indicate that fringes are formed by the superposition of two approximately parallel wave fronts. Wave fronts having the necessary characteristics can be obtained by mounting gratings on the arms of the interferometer in the places of the customary reflecting mirrors. The construction and action of such an interferometer has been studied; it is here described. The instrument becomes its own monochromator when the gratings are rotated to positions of minimum deviation for some particular wave-length. The customary form of the Michelson interferometer may be considered as the special case of the grating interferometer for which the grating space is large in comparison with λ.