You have now an exact image of the face of the sun and the few dark spots which are upon it, and we have brought, as it were, into our room that great globe of light and heat which sustains all the life and vigour upon our earth.
This small image can, however, tell us very little. Let us next see what photography can show us. The diagram (Fig. 46) shows a photograph of the sun taken by Mr. Selwyn in October 1860. Let me describe how this is done. You will remember that there is a point in the telescope tube where the rays of light form a real image of the object at which the telescope is pointed (see p. 44). Now an astronomer who wishes to take a photograph of the sun takes away the eye-piece of his telescope and puts a photographic plate in the tube exactly at the place where this real image is formed. He takes care to blacken the frame of the plate and shuts up this end of the telescope and the plate in a completely dark box, so that no diffused light from outside can reach it. Then he turns his telescope upon the sun that it may print its image.
But the sun's light is so strong that even in a second of time it would print a great deal too much, and all would be black and confused. To prevent this he has a strip of metal which slides across the tube of the telescope in front of the plate, and in the upper part of this strip a very fine slit is cut. Before he begins, he draws the metal up so that the slit is outside the tube and the solid portion within, and he fastens it in this position by a thread drawn through and tied to a bar outside. Then he turns his telescope on the sun, and as soon as he wishes to take the photograph he cuts the thread. The metal slides across the tube with a flash, the slit passing across it and out again below in the hundredth part of a second, and in that time the sun has printed through the slit the picture before you.
Fig. 46.
Photograph of the face of the sun, taken by Mr. Selwyn, October 1860, showing spots, faculæ, and mottled surface.
In it you will observe at least two things not visible on our card-image. The spots, though in a different position from where we see them to-day, look much the same, but round them we see also some bright streaks called faculæ, or torches, which often appear in any region where a spot is forming, while the whole face of the sun appears mottled with bright and darker spaces intermixed. Those of you who have the telescopes can see this mottling quite distinctly through them if you look at the sun. The bright points have been called by many names, and are now generally known as "light granules," as good a name, perhaps, as any other.
This is all our photograph can tell us, but the round disc there shown, which is called the photosphere, or light-giving sphere, is by no means the whole of the sun, though it is all we see daily with the naked eye. Whenever a total eclipse of the sun takes place—by the dark body of the moon coming between us and it, so as to shut out the whole of this disc—a brilliant white halo, called the crown or corona, is seen to extend for many thousands of miles all round the darkened globe. It varies very much in shape, sometimes forming a kind of irregular square, sometimes a circle with off-shoots, as in Fig. 47, which shows what Major Tennant saw in India during the total eclipse of August 18, 1868, and at other times it shoots out in long pearly white jets and sheets of light with dark spaces between. On the whole it varies periodically. At the time of few sun-spots its extensions are equatorial; but when the sun's face is much covered with spots, they are diagonal, stretching away from the spot-zones, but not nearly so far.
