The Boundary of the Solar Chromosphere

Abstract

THE question of the sudden ending of the chromosphere or its gradual fading away in accordance with Prof. Milne's theoretical views may not yet be settled finally by observation. Mr. R. W. Gurney is, however, under a misapprehension (NATURE, Feb. 16, p. 240) in thinking that the bright K line studied by Mr. P. A. Taylor and Mr. McCrea up to a height of nearly 100,000 km. above the sun's limb was thought to be an ordinary chromospheric line. The tangential slit happened to fall across a high prominence and the measures refer to the portions of the slit lying on the prominence, which gave a regular fading away with height; one or two obvious brightenings had to be ignored, where structure in the prominence complicated the issue. These points were easily recognised in the picture of the prominence shown in the second flash spectrum which was obtained with an objective prism.