New apparatus for observing magnetic fields on the surface of the sun has recently been put into use at the Hale Solar Laboratory in Pasadena. It utilizes the longitudinal Zeeman effect, wherein a spectral line is split by the field into two overlapping oppositely polarized components. The new equipment has three noteworthy features : (1) a large plane grating superior to those that have been available previously; (2) a photoelectric detector having two slits symmetrically placed on the wings of the chosen line, where the profile is steepest, with two photomultipliers connected to a difference amplifier; and (3) provision for scanning the sun's disk and mapping with a cathode-ray tube and a camera the intensity and polarity of the magnetic fields on the surface. The new plane grating used in the 75-foot underground Littrow spectrograph has a ruled area 13 by 20 cm with 600 grooves per mm, and is blazed in the fifth order green, where the dispersion is 1 1 mm per angstrom and the resolving power, measured photographically, is 600,000. Scattered light and ghosts are entirely negligible. The resolving power of this grating is ample to render line profiles with high accuracy, so that the interferometer, with its critical adjustments and low light transmission, can be eliminated.