The Solar Magnetograph.

A new instrument for measuring and recording weak magnetic fields on the surface of the sun has been developed for use with the 150-foot solar telescope and 75-foot spectrograph of the Hale Solar Laboratory. Principal features include: a superior grating of high resolving power for use in the fifth-order spectrum; an electro-o tic analyzer for polarization; a double-slit detector for the longitudinal Zeeman effect; and a self-sync ronous system by which the disk of the sun is scanned in a raster of parallel traces, the results as to magnetic intensity and polarity being presented conformally on the screen of a cathode-ray tube and recorded by a camera. The noise level (about 0.1 gauss) is such that fields of the order of 1 gauss can be recorded readily. The method of calibration is described, and the possibility is pointed out of using the instrument, with a slight optical modification, for studying small Doppler shifts in the sun's atmosphere.