Successive Diffractions by a Concave Grating

By placing one or more pairs of plane mirrors on the Rowland circle of a concave grating, adjusted so as to reflect the light back to the grating one or more times, dispersion and resolving power equivalent to a very high order of the grating may be achieved. The method is particularly advantageous when used with a strongly blazed grating. Actual tests with line and band spectra show a considerable increase over the theoretical resolving power for a single diffraction in the direction of blaze. Spectra diffracted as many as seven times are photographed, and show the ghost strength and freedom from overlapping characteristic of a single diffraction. Under suitable conditions, the astigmatism may be eliminated.