The first coudé spectrographs built at Mount Wilson by W. S. were systems employing a large prism. Gratings were used to an increasing extent, beginning in 1932. The development of Schmidt cameras in 1934, and of a 114-in. focus spherical-mirror camera in 1939, increased efficiency in the ultraviolet and made it possible to cover any region in the spectrum without changing focus or tilt. Photometric accessories have been developed to permit measuring profiles and integrated intensities of absorption lines. An exposure meter and automatic guiding have been developed by H. W. .

The distribution or intensity within stellar images has been measured photo-electrically under various conditions of seeing, as a guide to spectrograph design. The efficiency of a spectrograph can be increased by using large apertures for the collimator, grating and camera, by increasing angular dispersion and by using an image slicer. The relative speed has been calculated for various telescope-spectrograph systems and for different grades of seeing. In order to increase efficiency as far as possible it will be desirable to use multiple gratings of maximum aperture, perhaps with the combined width of ruling considerably greater than the length of grooves, and illuminated with an elliptical collimated beam from crossed cyclindrical mirrors.

Direct photo-electric scanning of stellar spectra can yield a useful level of accuracy with a 100-in. telescope, in a reasonable time of observation, for a resolving power of 50,000, to a magnitude 8; and for a resolving power of 500, to a magnitude 11. A Fabry-Perot interferometer may be used to advantage with a photo-electric detector for measuring the profiles of selected absorption lines, which can serve as standards for photographic photometry.