An instrument is described which, by the addition of two moving parts to the conventional form of comparator. enables the observer to read from the precision screw directly the values of the wave-lengths or wave numbers that would ordinarily be computed by the Hartmann formula. The extra moving parts are (1) a light carriage on geometric ball bearing ways to carry the plate and (2) a transfer mechanism functioning between that and the main carriage. The whole apparatus operates on a principle of geometric projection and is, in effect, a mechanical equivalent of the Hartmann formula. It is applicable to grating as well as prismatic spectra.
L. G. HOXTON and D. W. MANN, "A Direct Reading Comparator for Spectra," J. Opt. Soc. Am. 27, 150-154 (1937)