It will readily be seen from the explanation just given that if thegeneral background of the field has been made faint enough (by a high dispersion), it may be possible to open the slit so widely that the entire prominence may be seen in its true form, in either of the re gions, C or F, of the spectrum. The images seen in this way are, however, formed only by light of one particular wave length: if the point of the spectrum selected is that upon which there falls a line due to hydrogen, then the image seen will be that arising only from the hydrogen in the prominence; whatever other elements there may be will have no effect, and we shall, in fact, see the exact form and distri bution of that portion of the cloud which is composed of hydrogen alone.
A remarkable application of this principle is made in the so-called spectro-heliograph, an instrument invented by the Ameritan astron omen, G. F. Hale, and first successfully em ployed by him in the year 1891. Here all light is screened from the field except that coming through the very narrow slit of the spectro scope, this slit being sufficiently long to include the whole object observed, or even the entire sun's disc. By letting the sun's image sweep over the slit, every part is successively seen by the light of a single wave length, and thus we may view the distribution of the hydrogen solely, or of the calcium, or of the other ele ments upon or about the sun. Finally, by al lowing the very narrow, moving image of the slit to fall upon a photographic plate, we may obtain a photograph of the sun from a single selected element which it contains, provided only that there emanate from this element a line of sufficient brightness for this purpose.
The glowing prominences sometimes extend to great heights above the sun's surface: though the usual depth of the chromosphere is but from 5,000 to 10,000 miles, material has been observed at times of great eruptive dis turbance to extend to a distance of no less than 400,000 miles — a distance nearly equal to the radius of the sun itself. While, gener ally spealdng, all the elements of the sun are found in the lower regions of the chromo sphere, it is only the lighter ones which attain these enormous altitudes in the eruptive prom inences, and the opposite is true of the constitu tion of the great, comparatively quiescent chromospheric clouds which lie above the pho tosphere. Thus Jewell has found that in the high levels of the envelope there are hydrogen, helium, parheliutn, magnesium, sodium and ytter bium, while at the low levels are chromium, iron, cobalt, nickel, manganese, yttrium, cad mium, zinc, carbon and vanadium. Several
substances, notably calcium, give contradictory results, the average level being sometimes high and sometimes low; as only very minute quantities of some substances are required to produce spectral lines, however, the apparent discordance is not surprising.
From minute displacements of well-known lines in the spectrum (see SPECTRUM ANALY are inferred of the incandescent es from which these lines are proceeding. On this principle motions of the fixed stars have been determined. Thus Sirius is ap proaching us at the rate of 10 males per second, and Arcturus is approaching us at the rate of five miles per second. The principle applied to the results of the spectroscopic examination of the prominences of the chromosphere, as well as a direct observation of their apparent movement across the line of sight, shows that they are due to enormous outbursts of gases and vapors from the sun. These gases are sometimes projected outward or even tangen tially with extraordinary velocities, rates of mo tion so high as 300 miles a second having been reported. In some cases the apparent motion of a single cloud may be a perspective effect due to the successive formation of many clouds over an extended area, yet it is quite certain that in many of the cases the enormous veloci ties observed are real, and it is probable that they may be in part, at least, due to electrical action. Bet much is still to be learned, not only as to The true cause and movements of the eruptive prominences, but also in explanation of the clouds' which are nearly quiescent for a considerable length of time, though so near the enormously distributed region of the sun's photosphere.
The most recent, semi popular, but authoritative, work is 'The Sun,' by C. G. Abbot (1911) ; this contains many references to more detailed or extended publi cations. A larger and very important recent work is 'Physik der Sonne,' by E. Pringsheim (Leipzig 1910). Numerous papers will be found in the 'Proceedings' of the Royal So ciety (London), the thirophysical Journal (Chi cago), and in practically all journals devoted to general astronomy and to astronomical spectroscopy.