Total eclipse of the sun, as drawn by Major Tennant at Guntoor in India, August 18, 1868, showing corona and the protuberances seen at the beginning of totality.
And besides this corona there are seen very curious flaming projections on the edge of the sun, which begin to appear as soon as the moon covers the bright disc. In our diagram (Fig. 47) you see them on the left side where the moon is just creeping over the limits of the photosphere and shutting out the strong light of the sun as the eclipse becomes total. A very little later they are better seen on the other side just before the bright edge of the sun is uncovered as the moon passes on its way. These projections in the real sun are of a bright red colour, and they take on all manners of strange shapes, sometimes looking like ranges of fiery hills, sometimes like gigantic spikes and scimitars, sometimes even like branching fiery trees. They were called prominences before their nature was well understood, and will probably always keep that name. It would be far better, however, if some other name such as "glowing clouds" or "red jets" could be used, for there is now no doubt that they are jets of gases, chiefly hydrogen, constantly playing over the face of the sun, though only seen when his brighter light is quenched. They have been found to shoot up 20,000, 80,000, and even as much as 350,000 miles beyond the edge of the shining disc; and this last means that the flames were so gigantic that if they had started from our earth they would have reached beyond the moon. We shall see presently that astronomers are now able by the help of the spectroscope to see the prominences even when there is no eclipse, and we know them to be permanent parts of the bright globe.
This gives us at last the whole of the sun, so far as we know. There is, indeed, a strange faint zodiacal light, a kind of pearly glow seen after sunset or before sunrise extending far beyond the region of the corona; but we understand so little about this that we cannot be sure that it actually belongs to the sun.
And now how shall I best give you an idea of what little we do know about this great surging monster of light and heat which shines down upon us? You must give me all your attention, for I want to make the facts quite clear, that you may take a firm hold upon them.
Our first step is to question the sunlight which comes to us; and this we do with the spectroscope. Let me remind you how we read the story of light through this instrument. Taking in a narrow beam of light through a fine slit, we pass the beam through a lens to make the rays parallel, and then throw it upon a prism or row of prisms, so that each set of waves of coloured light coming through the slit is bent on its own road and makes an upright image of the slit on any screen or telescope put to receive it (see Fig. 21, p. 52). Now when the light we examine comes from a glowing solid, like white-hot iron, or a glowing liquid, or a gas under such enormous pressure that it behaves like a liquid, then the images of the slit always overlap each other, so that we see a continuous unbroken band of colour. However much you spread out the light you can never break up or separate the spectrum in any part.[2] But when you send the light, of a glowing gas such as hydrogen through the spectroscope, or of a substance melted into gas or vapour, such as sodium or iron vaporised by great heat, then it is a different story. Such gases give only a certain number of bright lines quite separate from each other on the dark background, and each kind of gas gives its own peculiar lines; so that even when several are glowing together there is no confusion, but when you look at them through the spectroscope you can detect the presence of each gas by its own lines in the spectrum.
Plate I.
To make quite sure of this we will close the shutters and put a pinch of salt in a spirit-flame. Salt is chloride of sodium, and in the flame the sodium glows with a bright yellow light. Look at this light through your small direct-vision spectroscopes[3] and you see at once the bright yellow double-line of sodium (No. 3, Plate I.) start into view across the faint continuous spectrum given by the spirit-flame. Next I will show you glowing hydrogen. I have here a glass tube containing hydrogen, so arranged that by connecting two wires fastened to it with the induction coil of our electric battery it will soon glow with a bright red colour. Look at this through your spectroscopes and you will see three bright lines, one red, one greenish blue, and one indigo blue, standing out on the dark background (No. 4, Plate I.)