and ordinary observers will not see much, where professed astronomers have usually found little. But as these pages may possibly fall into the hands of some, whose advantages or enterprise may lead them to attack a neglected object, the following points may be specified.

1. The Phases. These will be easily seen, and are only remarkable because the breadth of the enlightened part has been sometimes found less than it should have been from calculation. Schröter noticed this: Beer and Mädler confirm it, but their explanation of a dense atmosphere enfeebling the terminator, or boundary of light and darkness, is inadequate, as their observation was before sunrise, when even the dullest part of the disc would be very luminous. See this again in Venus.

2. The Mountains. At the close of the last and beginning of this century, Schröter, of Lilienthal in Hanover, a most diligent observer, and his assistant Harding, obtained what they deemed sufficient evidence of a mountainous surface in the occasional blunting of the S. horn,* some minute projections on its outer edge, and an irregular curve of the terminator; they gave the inferred elevations a height of nearly 11 miles perpendicular.

3. The Atmosphere. The decrease of light towards the terminator, and the occasional presence of dark streaks and spots, indicated to the same astronomers a vaporous envelope, where they thought they even saw traces of the action of winds. From a combination of various appearances they deduced a rotation in 24h om 53s on an axis inclined about 70° to the ecliptic of Mercury. A spot with faint lines diverging from it, NE. and S. was seen by Prince a little S. of the centre, in unusually clear air, 1867, June 11.* Birmingham also was pretty

* Noble also has suspected this (1864).

certain that there was a large white spot near E. limb, 1870, March 13. In De La. Rue's magnificent Newtonian, 10 f. focus and 13 in. aperture, constructed by himself, the planet has a rosy tinge. Secchi has remarked that the disc is always very ill-terminated, and very faint at the edges.

Transits of Mercury are comparatively frequent; they will be visible in Europe in 1878 and 1894. The planet breaks in upon the Sun as a dark notch, sometimes preceded, it is said, by a penumbral shade; but the earliest impression will be missed, unless the exact point of the Sun's limb is known, and kept central in the field. As it advances, the part of Mercury not yet entered may be rendered visible by being projected upon the 'corona,' which is so conspicuous in total solar eclipses, and has been known to relieve dark bodies in front of it, such as an inferior planet, or even a portion of the Moon. On finally entering the Sun, or beginning to leave it, the planet has been seen lengthened towards the limb from irradiation fully on the disc, where Mercury appears intensely black, some astronomers have given it a slight dusky border, others a narrow luminous ring: both, probably, deceptions from the violent contrast and the fatigue of the eye, especially as others have remarked nothing of the kind. A similar explanation may be applied to a whitish or grey spot on the dark planet, seen by Wurzelbauer, 1697, Schröter, Harding, and Kohler, 1799; Fritsch and others, 1802; Moll and his assistants, 1832 (when Harding clearly distinguished two spots, and Gruithuisen suspected one); and recognised in England and America, 1848, and by Huggins and Elger, 1868 when Browning perceived two spots again. No terrestrial analogy will explain a luminosity thus visible close to the splendour of the Sun; and it seems natural to refer it to

the exhausted state of the retina: an artificial disc, however, subsequently tried by Huggins, shewed nothing of the kind. And everything that is seen, however improbable, should always be recorded: an opposite procedure would have effectually precluded many an important discovery. We may remark that Schröter and Harding ascribed to these spots a motion corresponding with the rotation which they subsequently inferred from other indications.*

VENUS.

Fairest of stars, last in the train of night,
If better thou belong not to the dawn,

Sure pledge of day, that crown'st the smiling morn
With thy bright circlet, praise HIM in thy sphere.

MILTON.

THE most beautiful of all the heavenly bodies to the unaided eye is often a source of disappointment in the telescope. For the most part it resists all questioning beyond that of Galileo, to whom its phases revealed the confirmation of the Copernican theory; an important discovery, which he involved for a season in the following ingenious Latin transposition:

Hæc immatura à me jam frustra leguntur, o. y.

the letters in their original order forming the words Cynthiæ figuras æmulatur mater amorum.

Observers in general will subscribe to the experience of Herschel II., who says it is the most difficult of all the planets

* A similar phænomenon was observed on Venus in the transit of 1761 (Append. ad Ephem. Astron. 1766, 62).

'The intense lustre of its illumi

6

to define with telescopes. nated part dazzles the sight, and exaggerates every imperfection of the telescope: yet we see clearly that its surface is not mottled over with permanent spots like the Moon; we notice in it neither mountains nor shadows, but a uniform brightness, in which sometimes we may indeed fancy, or perhaps more than fancy, brighter or obscurer portions, but can seldom or never rest fully satisfied of the fact;' and he infers, like his father, and Huygens long before, that we do not see, as in the Moon, the real surface of these planets' (Venus and Mercury) but only their atmospheres, much loaded with clouds, and which may serve to mitigate the otherwise intense glare of their sunshine.'* Notwithstanding, however, the authority of this opinion, perseverance has of late years rendered it at least very doubtful: and moderate-sized instruments have proved occasionally of service. Further observations of this planet are much needed, aud would probably be rewarded by interesting results. The following are the chief points to be noticed :

1. The Phases. These are easily seen, and very beautiful, excepting the dull gibbous form in the superior or further part of the orbit, where the disc is also small; near the greatest elongation from the Sun towards the E. when she is an evening, or W. when a morning star, Venus puts on a beautiful shape that of the moon in quadrature; between these points in the inferior or nearer part of the orbit, she is a most elegant

* Chacornac is of the same opinion, from the absence of polarization in the light of Venus. His apparatus shews her brightness to be Io times greater than that of the most luminous parts of the Moon. The reduction of her light to perceptibly diminished the image from removal of irradiation (?). Jupiter the same.

*

crescent, larger, sharper, and thinner in proportion as she is nearer really to the Earth and apparently to the Sun. This crescent has been seen even with the naked eye in the sky of Chile, and with a dark glass in Persia, and a very small telescope will shew it. When quite slender it should not be looked for after sunset or before sunrise, as it lies too low in the vapour; but an equatorial stand will find it in the middle of the day-an exquisite object; and thus, or in a transit instrument, it has been many times traced as a mere curved thread extremely near the Sun.† Ordinary observers may succeed in seeing it a very delicate crescent soon after it has passed its inferior conjunction, by watching for its earliest appearance

Under the opening eyelids of the morn,'

setting the finder, or the telescope with the lowest power, upon it, and following it at intervals sufficient to keep it in the field, till it has cleared the vapours of the horizon; in this way it may be readily viewed for hours. In fact, the planet is best seen for many purposes in the day-time; its light, unpleasantly dazzling in a dark sky, so as to bear a screen-glass, is subdued by day to a beautiful pearly lustre. Nor is it very difficult to find. For some time about its greatest brightness, at 40° from the Sun in the inferior part of the orbit, it not merely casts a shade by night, but is visible to the naked eye at noon-day, provided its position is pretty well known. A little careful steady gazing will then bring it out as an intense white point, and, if the air is good, it will be a charming telescopic object.

*Theodore Parker saw it in America when 12 years old, and ignorant of its existence, and when no one else could perceive it.

Dawes considers that, with due precaution, Venus might be seen in her upper conjunction within 1' of the Sun's limb.