Fig. 50.
Bright lines of prominences.
Sun-spectrum with dark lines.
Wherever the telescope is turned all round the sun the lines of luminous gas are seen, showing that they form a complete layer outside the photosphere, or light-giving mass, of the sun. This layer of luminous gases is called the chromosphere, or coloured sphere. It lies between the photosphere and the corona, and is supposed to be at least 5000 miles deep, while, as we have seen, the flames shoot up from it to fabulous heights.
The quiet red flames are found to be composed of hydrogen and another new metal called helium; but lower down, near the sun's edge, other bright lines are seen, showing that sodium, magnesium, and other metals are there, and when violent eruptions occur these often surge up and mingle with the purer gas above. At other times the eruptions below fling the red flames aloft with marvellous force, as when Professor Young saw a long low-lying cloud of hydrogen, 100,000 miles long, blown into shreds and flung up to a height of 200,000 miles, when the fragments streamed away and vanished in two hours. Yet all these violent commotions and storms are unseen by us on earth unless we look through our magic glasses.
You will wonder no doubt how the spectroscope can show the height and the shape of the flames. I will explain to you, and I hope to show them you one day. You must remember that the telescope makes a small real image of the flame at its focus, just as in one of our earlier experiments you saw the exact image of the candle-flame upside down on the paper (see p. 33). The reason why we only see a strip of the flame in the spectroscope is because the slit is so narrow. But when once the sunlight was dispersed so as no longer to interfere, Dr. Huggins found that it is possible to open the slit wide enough to take in the image of the whole flame, and then, by turning the spectroscope so as to bring one of the bright hydrogen lines into view, the actual shape of the prominence is seen, only it will look a different colour, either red, greenish-blue, or indigo-blue, according to the line chosen. As the image of the whole sun and its appendages in the telescope is so very small, you will understand that even a very narrow slit will really take in a very large prominence several thousand miles in length. Fig 51 shows a drawing by Mr. Lockyer of a group of flames he observed very soon after Dr. Huggins suggested the open slit, and these shapes did not last long, for in another picture he drew ten minutes later their appearance had already changed.
Fig. 51.
Red prominences, as drawn by Mr. Lockyer during the total eclipse of March 14, 1869.