fectly round well-defined planetary disc, surrounded by two, three, or more alternately dark and bright rings, which, if examined attentively, are seen to be slightly coloured at their borders. They succeed each other nearly at equal intervals round the central disc, and are usually much better seen, and more regularly and perfectly formed, in refracting than in reflecting telescopes. The central disc too is much larger in the former than in the latter description of telescope. These discs were first noticed by Sir William Herschel, who first applied sufficiently high magnifying powers to telescopes to render them visible. They are not the real bodies of the stars, which are infinitely too remote to be ever visible with any magnifiers we can apply ; but spurious or unreal images, resulting from optical causes, which are still to a certain degree obscure." The various appearances of stars,as seen in telescopes, particularly the resolution of stars which appear single into two or more, render them excellent objects, when classified, for the examination of the power and goodness of these Instruments. Such a classification was made by Sir J. Herschel (' 31em. Astron. Soc.') ; and the paper is reprinted at the end of the explanation (pub lished separately) of the maps of the stars published by the Society for the Diffusion of Useful Knowledge.
The magnitude of a star is a notion formed by observers as to the apparent quantity of light which comes from them, on which they are divided into classes. Those which are visible to the naked eye are usually divided Into six magnitudes, which„according to W. Herschel, emit quantities of light which are (roughly) in about the proportions of the numbers 100, 25,12, 6, 2, and 1. But though practical astronomers are tolerably well agreed as to the mode of naming most of the prin cipal stars in respect of magnitude, there are many about which they differ, and some as to which it is tolerably well known that the order of magnitude which adherence to old catalogues still procures for them, is not that which would have been given had they been new stars named in our day. The magnitudes of stars are in fact rather inde terminate after the first and second. An astronomer would hardly say that an appearance was like a star of the " first or second " magnitude; the difference of the two is too well established, though as to the fainter stars of the first magnitude, and the brighter ones of the second, there may be little to choose between them. But it is very common to speak of an appearance as being of the "second or third," "third or fourth," &c., magnitude, showing that the distinction between one magnitude and the next is not then very prominent. Sir John Herschel and Professor Struve, the two most assiduous observers of small magnitudes, usually differ (' 3fem. Astron. Soc.,' vol. iii. p. 180)
about a magnitude in their estimation of one star with another, from and below Struve's fourth or Herschel's fifth magnitude, down to Struve's twelfth or Herschel's thirteenth. When therefore the reader, who is no astronomer, hears of the constant reference to stars of all magnitudes down to the sixteenth, he must look upon it as a rough mode of estimating the relative brilliancies of the stars, in which a numerical nomenclature is far from being held to imply numerical accuracy.
Some stars (perhaps all) are variable in their magnitudes, and with periodical regularity, which is perhaps to be attributed to the effect of revolution round their axes ; it being imaginable that different parts of a star should give different kinds or quantities of light, either or both. [VARIABLE STARS.] There are a few instances in which a sudden appearance of a new star is recorded, followed, after a time, by its disappearance. Such a phenomenon made an astronomer, it is said, of Hipparchus ; and certainly the star which appeared in Cassiopeia in 1572 was the intro duction of Tycho Brah6 to the character of a public astronomer. [Biting, Tircuo, in BIOG. DIV.] Tycho Brah6 himself thought, from historical evidence, that a star bad appeared in Cassiopeia in 945 and 1264, that of his own time being in 1572, from which, if the historical evidence be correct, a new star might be expected to appear in that constellation in 1872 or thereabouts. But, on examining his evidence, we find it exceedingly vague and deficient in antiquity. (' Comp. to Maps of the Stars, p. 86.) It has been reasonably supposed that those stars which have most motion are comparatively near to the earth, and when it was requisite to choose a double star for the determination of the question of PARALLAX, 61 Cygni was selected, as being a star with a large proper motion; in fact, its right ascension alters yearly 5" '46, and its declination 3" .19. The experiment turned out favourably, and the parallax was discovered, and with it (roughly) the distance of the star from the solar system. And though light takes more than ten years to travel from this star to the earth, at the rate of two hundred thousand miles a second; yet, so far from this being anything enormous, it rather cuts down the idea which was entertained of the distance of these bodies. The absence of all parallax, in spite of repeated efforts to obtain it, made many speculations upon the possibility of the nearest starlight being hundreds of years in reaching us. Among other stars which have a decided proper motion, we may notice Sirius, Procyon, 61 Virginia, a Bootis, A Ophiuchi, p Ophiuchi, and 14 Cassiopeia.