SOLAR ECLIPSE, 1882.
From drawing by W. H. Wesley, Secretary R.A.S.; showing the prominences, the corona, and an unknown comet.
On September 19th, 1868, eclipse spectroscopy began with the Indian eclipse, in which all observers found that the red prominences showed a bright line spectrum, indicating the presence of hydrogen and other gases. So bright was it that Jansen exclaimed: “Je verrai ces lignes-là en dehors des éclipses.” And the next day he observed the lines at the edge of the uneclipsed sun. Huggins had suggested this observation in February, 1868, his idea being to use prisms of such great dispersive power that the continuous spectrum reflected by our atmosphere should be greatly weakened, while a bright line would suffer no diminution by the high dispersion. On October 20th Lockyer,[[10]] having news of the eclipse, but not of Jansen’s observations the day after, was able to see these lines. This was a splendid performance, for it enabled the prominences to be observed, not only during eclipses, but every day. Moreover, the next year Huggins was able, by using a wide slit, to see the whole of a prominence and note its shape. Prominences are classified, according to their form, into “flame” and “cloud” prominences, the spectrum of the latter showing calcium, hydrogen, and helium; that of the former including a number of metals.
The D line of sodium is a double line, and in the same eclipse (1868) an orange line was noticed which was afterwards found to lie close to the two components of the D line. It did not correspond with any known terrestrial element, and the unknown element was called “helium.” It was not until 1895 that Sir William Ramsay found this element as a gas in the mineral cleavite.
The spectrum of the corona is partly continuous, indicating light reflected from the sun’s body. But it also shows a green line corresponding with no known terrestrial element, and the name “coronium” has been given to the substance causing it.
A vast number of facts have been added to our knowledge about the sun by photography and the spectroscope. Speculations and hypotheses in plenty have been offered, but it may be long before we have a complete theory evolved to explain all the phenomena of the storm-swept metallic atmosphere of the sun.
The proceedings of scientific societies teem with such facts and “working hypotheses,” and the best of them have been collected by Miss Clerke in her History of Astronomy during the Nineteenth Century. As to established facts, we learn from the spectroscopic researches (1) that the continuous spectrum is derived from the photosphere or solar gaseous material compressed almost to liquid consistency; (2) that the reversing layer surrounds it and gives rise to black lines in the spectrum; that the chromosphere surrounds this, is composed mainly of hydrogen, and is the cause of the red prominences in eclipses; and that the gaseous corona surrounds all of these, and extends to vast distances outside the sun’s visible surface.
FOOTNOTES:
[1] Rosa Ursina, by C. Scheiner, fol.; Bracciani, 1630.