1. XII. An improved form of Littrow spectroscope
2. XXX. On the application of interference methods to spectroscopic measurements.—II
3. Kempe. ‘Lecture on Linkages’ 40–40.
4. 1881. Zeit. für Instrumentenkunde, November Since writing the above my attention has been called to an article infordescribing this particular “direct-vision” arrangement of the prism and mirror. There was, however, no indication of the general class of which this is but a particular type, and hence, of course, no indication of the conditions which it was necessary to fulfil to prevent a lateral shifting of the beam; indeed, the author seems to accept this lateral shifting as a necessary condition, for he says “Der im Minimum der Ablenkung durchgehende Strahl wird also bei dieser Anordnung nur seitlich etwas verschoben.” Professor Langley had also quite independently used this particular arrangement of prism and reflecting mirror in a modification of Foucault's “Lifting Prism” for separating different orders of superposed grating-spectra. In his use of it the lateral displacement was recognized as objectionable, and was mechanically corrected for by an ingenious arrangement, designed by Mr. C. T. Child, then assistant in the Observatory, which imparted to the mirror a small angular motion, just sufficient to correct for the angular displacement of the spectral image, as the whole system travelled down through the spectrum