Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/860

tion of matter in the upper regions of our atmosphere, no matter under what circumstances illuminated. Later, indeed, it was found that photographs taken at Java gave precisely the same features, so that, as Col. Tennant remarks in the latest communication on the subject (Monthly Notices of the Astronomical Society for June 1873), no one now supposes the corona to be other than a

solar phenomenon.

The accompanying picture (fig. 32), carefully copied from the first of Col. Tennant s series of photographs, will afford an idea of the nature of the corona s structure, though only an inadequate one, since photography can present only a portion of an object like the corona, whose light is for the most part extremely delicate.

Fig. 32.—Solar Corona.

Another very curious phenomenon connected with the sun is the faint nebulous aurora which accompanies him, known by the name of the Zodiacal Light. This pheno menon was first observed by Kepler, who described its appearance with sufficient accuracy, and supposed it to be the atmosphere of the sun. Dominic Cassini, however, to whom its discover} has been generally but erroneously attributed, was the first who observed it attentively, and gave it the name which it now bears. It is visible imme diately before sunrise, or after sunset, in the place where the sun is about to appear, or where he has just quitted the horizon. It has a flat lenticular form, and is placed obliquely on the horizon, the apex extending to a great distance in the heavens. Its direction is in general nearly in the plane of the sun s equator, and for this reason it is scarcely visible in our latitudes, excepting at particular seasons, when that plane is nearly perpendicular to the horizon. When its inclination is great, it is either con cealed altogether under the horizon, or it rises so little above it that its splendour is effaced by the atmosphere of the earth. The most favourable time for observing it is about the beginning of March, or towards the ver"- nal equinox. The line of the equinoxes is then situated in the horizon, and the arc of the ecliptic is more elevated than the equator by an angle of 23 J; so that the solar equator, which is slightly inclined to the ecliptic, approaches nearer to the perpendicular to the horizon, and the pyramid of the zodiacal light is consequently directed to a point nearer the zenith than at any other season of the year.

Numerous opinions have been entertained respecting the nature and cause of this singular phenomenon. Cassini thought it might be occasioned by the confused light of an innumerable multitude of little planets circulating round the sun, in the same manner as the Milky Way owes its appearance to the light of agglomerated myriads of stars. Its resemblance to the tails of comets has been noticed by Cassini and others. Mairan, following Kepler, ascribed it to the atmosphere of the sun. This hypothesis was gene rally adopted until Laplace showed it to be untenable, because no atmosphere could rotate with the sun if extend ing beyond the distance where centrifugal force is balanced by gravity, and this distance lies far within the observed range of the zodiacal light.

The general opinion respecting the zodiacal light at the present day is, that it forms the outer part of the solar corona, so that if the light of the sun could be for a time obliterated without rendering his appendages invisible, we should see the corona as shown in the preceding figure, merging gradually into the faint glow of the zodiacal light. It must not be understood, however, that this theory in any sense implies a continuity of substance between the zodiacal light and the corona, any more than what has been discovered respecting the corona implies that the corona is a true solar atmosphere. All that is to be understood is, that (1) the inner corona, (2) the outer radiated corona, and (3) the zodiacal light, form part of the series of appendages surrounding the orb of the sun.

It is to be noted, indeed, that the true atmosphere of the sun appears to underlie even the sierra. It had been noted by Secchi in 18G9, that on the very limb of the sun the solar spectrum becomes continuous; and he inferred that there must exist an atmosphere (relatively very shallow), consisting of the vapours which occasion the dark lines of the solar spectrum, and that the brightness of the lines of these vapours corresponding very nearly to the brightness of the ordinary solar spectrum for the parts of the sun very near to the edge, the dark lines of the latter spectrum are as it were cancelled, and so a continuous spectrum is pro duced. This view was shown during the eclipse of December 1870 to be perfectly correct; for Prof. Young, directing his analysing spectroscope to the part of the sun s lirnb which was to disappear last, found that at the instant when totality commenced, the solar spectrum was suddenly replaced by a spectrum consisting of thousands of bright lines, that being precisely the kind of spectrum which Secchi s theory required. This observation was success fully renewed during the eclipse of December 1871, and again during the annular eclipse of June 1872. On the latter occasion the remarkable circumstance was noted, that while the bright-line spectrum was only visible for a second or two on the eastern limb (just before the formation of the ring), it was visible for six or seven seconds on the western limb (just after the annulus was broken).

Chapter VIII.—The Inferior Planets Mercury and Venus.

Mercury is a small body, but emits a very bright white Mercury light, though he is seldom to be seen, always keeping near the sun.

Mercury is about 3060 English miles in diameter, and

ais mean distance from the sun about 35 millions of miles. On account of his smallness and brilliancy, it is extremely difficult to find any spot on his disk so distinctly marked as to afford the means of determining his rotation. AH attentive observation of the variations of the phases of Mercury led Schroter to infer that the planet revolves about its axis in a period of 24 hours 5 min. 30 sec. M. Harding discovered in 1801 an obscure streak on the

southern hemisphere of the planet, the observations of