THE spectrum of the solar chromosphere may be observed at a time of total eclipse by various methods. Frequently the thin crescent of the solar gases is photographed through a prismatic camera, the crescent itself acting as slit. In 1898, Dr. W. W. Campbell devised an important modification of this method of observing the flash spectrum. It consisted in the introduction of two new features, (1) a wide slit in the focal plane of the camera perpendicular to the dispersion and so to the monochromatic image-crescents, (2) a moving photographic plate moved during the exposure in its own plane in a direction perpendicular to the slit. The slit permits a short length (of the order of 1 3/2inch) of the central portion of each crescent to fall on the plate.