The hydrogen lines on the sun's disk are frequently distorted by the rapid motion of the gas toward or from the observer. For example, a mass of hydrogen descending into a vortex above a sunspot is indicated by a local displacement of the lines, which frequently indicate a velocity of 6okm. sec. In such a case the distorted portion of the line would fall outside the narrow slit of the spectroheliograph, and the image of the rapidly moving gas would not be recorded. The spectrohelioscope not only brings these moving gases into view but also indicates their velocity. A "line-shif ter," consisting of a plane parallel-sided glass plate mounted behind the second slit, displaces the line toward red or violet when rotated. In all regions where he suspects motion, the observer frequently shifts the line on the slit, thus "tuning in" so as to pick up the wave-lengths as altered by the motion of the luminous gases. In this way, for example, a rapidly ascending mass of hydrogen, near the middle of the sun (revealed by a line outside the Ha line on its violet side), may be watched as it shoots upward, curves over in a long arch nearly parallel for some distance with the surface (its line shown near the centre of the Ha line), and descends in a continuous arch toward the sun or is caught in a vortex and whirled downward (the line being seen outside the Ha line on its red side). A divided arc
attached to the line-shifter gives the velocity of the gas.
The spectrohelioscope affords a rapid means of detecting and analysing eruptions or other important solar phenomena, as the entire limb and disk can be examined in a few minutes. Eight or more of these instruments will soon be used systematically at as many solar observatories, distributed around the world, thus permitting the sun's atmosphere to be kept under nearly constant observation. In this way it is hoped to increase our knowledge of the connection between solar eruptions and such geophysical phenomena as the aurora, the magnetic storm and certain interruptions in radio transmission, which probably depend upon the bombardment of the earth's atmosphere by electrified particles projected from the sun.
Descriptions of spectroheliographs and results by Hale, Deslandres, Evershed, Newall and others have appeared in various papers in Astronomy and Astrophysics, Astrophysical Journal, Comptes rendus, Bulletin astronomique, Monthly Notices of the Royal Astronomical Society and the publications of the Yerkes, Meudon, Kodaikanal, Cambridge and Mt. Wilson observatories. Papers on the spectro helioscope may be found in the Publications of the National Academy of Sciences, Nature, Astrophysical Journal and Publications of the Astronomical Society of the Pacific. (G. E. H.)