To accomplish this a light framework may be constructed in the shape of a truncated cone. At its narrow end it slips or screws on to the eye-end of the telescope, and it may be made of any length required, in proportion to the size of solar disc which it is desired to obtain. It should be covered with black cloth, and its base may be a board with white paper stretched on it to receive the image, which is viewed through a small door in the side. In place of the board with white paper, other expedients may be tried. Noble recommends a surface of plaster of Paris, smoothed while wet on plate glass, and this is very good if you can get the plaster smooth enough. I have found white paint, laid pretty thickly on glass and then rubbed down to a smooth matt surface by means of cuttle-fish bone, give very satisfactory results. Should it be desired to exhibit the sun's image to several people at once, this can easily be done by projecting it upon a sheet of paper fastened on a drawing-board, and supported at right angles to the telescope by an easel. The framework, or whatever takes its place, being in position, the telescope is pointed at the sun by means of its shadow; when this is perfectly round, or when the shadow of the framework perfectly corresponds to the shape of its larger end, the sun's image should be in the field of view.
CHAPTER IV
THE SUN'S SURROUNDINGS
We have now reached the point beyond which mere telescopic power will not carry us, a point as definite for the largest instrument as for the smallest. We have traced what can be seen on the visible sun, but beyond the familiar disc, and invisible at ordinary seasons or with purely telescopic means, there lie several solar features of the utmost interest and beauty, the study of which very considerably modifies our conception of the structure of our system's ruler. These features are only revealed in all their glory and wonder during the fleeting moments in which a total eclipse is central to any particular portion of the earth's surface.
A solar eclipse is caused by the fact that the moon, in her revolution round the earth, comes at certain periods between us and the sun, and obscures the light of the latter body either partially or totally. Owing to the fact that the plane of the orbit in which the moon revolves round the earth does not coincide with that in which the earth revolves round the sun, the eclipse is generally only partial, the moon not occupying the exact line between the centres of the sun and the earth. The dark body of the moon then appears to cut off a certain portion, larger or smaller, of the sun's light; but none of the extraordinary phenomena to be presently described are witnessed. Even during a partial eclipse, however, the observer may find considerable interest in watching the outline of the dark moon, as projected upon the bright background of the sun. It is frequently jagged or serrated, the projections indicating the existence, on the margin of the lunar globe, of lofty mountain ranges.
FIG. 19.—ECLIPSES OF THE SUN AND MOON.
Occasionally the conditions are such that the moon comes centrally between the earth and the sun (Fig. 19), and then an eclipse occurs which may be either total or annular. The proportion between the respective distances from us of the sun and the moon is such that these two bodies, so vastly different in real bulk, are sensibly the same in apparent diameter, so that a very slight modification of the moon's distance is sufficient to reduce her diameter below that of the sun. The lunar orbit is not quite circular, but has a small eccentricity. It may therefore happen that an eclipse occurs when the moon is nearest the earth, at which point she will cover the sun's disc with a little to spare; or the eclipse may occur when she is furthest away from the earth, in which case the lunar diameter will appear less than that of the sun, and the eclipse will be only an annular one, and a bright ring or 'annulus' of sunlight will be seen surrounding the dark body of the moon at the time when the eclipse is central.
All conditions being favourable, however—that is to say, the eclipse being central, and the moon at such a position in her orbit as to present a diameter equal to, or slightly greater than, that of the sun—a picture of extraordinary beauty and wonder reveals itself the moment that totality has been established. The centre of the view is the black disc of the moon. From behind it on every side there streams out a wonderful halo of silvery light which in some of its furthest streamers may sometimes extend to a distance of several million miles. In the Indian Eclipse of 1898, for example, one streamer was photographed by Mrs. Maunder, which extended to nearly six diameters from the limb of the eclipsed sun (Plate [VIII.]). The structure of this silvery halo is of the most remarkable complexity, and appears to be subject to continual variations, which have already been ascertained to be to some extent periodical and in sympathy with the sun-spot period. At its inner margin this halo rests upon a ring of crimson fire which extends completely round the sun, and throws up here and there great jets or waves, which frequently assume the most fantastic forms and rise to heights varying from 20,000 to 100,000 miles, or in extreme instances to a still greater height. To these appearances astronomers have given the names of the Corona, the Chromosphere, and the Prominences. The halo of silvery light is the Corona, the ring of crimson fire the Chromosphere, and the jets or waves are the Prominences.