483. For the first requirement it is only essential that the instrument should be so arranged that it can command every portion of the sky. This may be accomplished in various ways: the best method of accomplishing it is shown in Plate XII., which represents an eightinch telescope, equatorially mounted-or, shortly, an equatorial—that is, an instrument so mounted that a heavenly body may be followed from rising to setting by one continuous motion of the telescope, which motion may be communicated by clockwork.

484. In this arrangement a strong iron pillar supports a head-piece, in which is fixed the polar axis of the instrument parallel to the axis of the Earth, which polar axis is made to turn round once in twenty-four hours by the clock shown to the right of the pillar.

485. It is obvious that a telescope attached to such an axis will always move in a circle of declination, and that a clock, carrying the telescope in one direction as fast as the Earth is carrying the telescope from a heavenly body in the opposite one, will keep the telescope fixed on the object. It is inconvenient to attach the telescope directly to the polar axis, as the range is then limited it is fixed, therefore, to a declination axis, placed above the polar axis, and at right angles to it, as shown in the plate.

486. For the other kinds of work, telescopes, generally of small power except in important observatories, are mounted as altazimuths, transit-instruments, transitcircles, and zenith-sectors. These descriptions of mounting, and their uses, will be described in Chap. VII.

LESSON XXXIX,- THE SOLAR

SPECTRUM. THE

SPECTROSCOPE. KIRCHHOFF'S DISCOVERY. PHYSICAL CONSTITUTION OF THE SUN.

487. A careful examination of the solar spectrum has told us the secret of the enormous importance of solar radiation (Art. 124). Not only may we liken the gloriously coloured bands which we call the spectrum to the keyboard of an organ—each ray a note, each variation in colour a variation in pitch-but as there are sounds in nature which we cannot hear, so there are rays in the sunbeam which we cannot see.

488. What we do see is a band of colour stretching from red, through yellow, green, blue, violet, indigo, to lavender, but at either end the spectrum is continued. There are dark rays before we get to the red, and other dark rays after we leave the lavender-the former heat rays, the latter chemical rays; and this accounts for the threefold action of the sunbeam: heating power, lighting power, and chemical power.

489. When a cool body, such as a poker, is heated in the fire, the rays it first emits are entirely invisible, or dark if we looked at it through a prism, we should see nothing, although we can easily perceive by the hand that it is radiating heat. As it is more highly heated, the radiation from the poker gradually increases, until it becomes of a dull red colour, the first sign of incandescence; in addition to the dark rays it had previously emitted, it now sends forth waves of red light, which a prism will show at the red end of the spectrum: if we still increase the heat and continue to look through the prism, we find, added to the red, orange, then yellow, then green,

then blue, indigo, and violet, and when the poker is whitehot all the colours of the spectrum are present. If, after this point has been reached, the substance allows of still increased heating, it will give out with increasing intensity the rays beyond the violet, until the glowing body can rapidly act in forming chemical combinations, a process which requires rays of the highest refrangibility -the so-called chemical, actinic, or ultra-violet rays.

490. We owe the discovery of the prismatic spectrum to Sir Isaac Newton, but the beautiful colouring is but one part of it. Dr. Wollaston in the year 1802 discovered that there were dark lines crossing the spectrum in different places. These have been called Fraunhofer's lines, as an eminent German optician of that name afterwards mapped the plainest of them with great care : he also discovered that there were similar lines in the spectra of the stars. The explanation of these dark lines we owe to Stokes, and more particularly to Kirchhoff. The law which explains them was, however, first proved by Balfour Stewart.

491. We shall observe the lines best if we make our sunbeam pass through an instrument called a spectroscope, in which several prisms are mounted in a most careful manner. We find the spectrum crossed at right angles to its length by numerous dark lines-gapswhich we may compare to silent notes on an organ. Now if we light a match and observe its spectrum, we find that it is continuous—that is, from red through the whole gamut of colour to the visible limit of the violet: there are no gaps, no silent notes, no dark lines, breaking up the band.

Another experiment. Let us burn something which does not burn white; some of the metals will answer our once by the brilliant colours that

purpose. We see at

fall upon our eye from the vivid flame that we have here