Examples of High Resolution Obtainable by Double Passing a High Quality Grating*

Critical resolution tests were made near limiting geometric incidence angles on a 7.25-in. wide 300-grooves per mm Bausch & Lomb grating. The importance of double passing a grating to increase the available resolving power is discussed and exhibited by reproductions of the hyperfine structure of Hg λ4358, λ5461, λ15 295. A photograph of a reversal in Hg λ4358 presents a resolving power well in excess of 1 000 000 which far exceeds the grazing incidence resolving power of the grating used single pass. An analysis of the hyperfine structure of Hg λ15 295 is given. A discussion of ghost intensities in going from single pass to double pass is also presented. Maximal performance at 4 widely separated wavelengths is given.