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Venus as seen by these astronomers, present very defined outlines of spots, none of which can now be traced. Whatever peculiarities were formerly noticed, nothing is more certain now, than that it is one of the most difficult of all the planets to define with a telescope; notwithstanding this, whether it be contemplated with the unassisted sight, shining in the eye of day in partnership with the sun, and sending forth a trembling flood of radiance as the morning or evening star, or examined as a telescopic object, and traced through all its various phases of a minature moon, it is an inexpressibly lovely object, and one that will always excite admiration.
Sir William Herschel has observed Venus when about one-third illuminated, and noticed a blaish shading towards the central part of the disc; this dark shading was in the form of two faintly defined semi-circles, meeting together in an angle near the boundary of light and darkness; one of these circular shadings was about a third, the other a fourth of the breadth of the planet. This astronomer also observed, that Venus was much brighter round her limb, than in that part which separates the enlightened from the obscure parts of her disc. (See Plate, fig. 1.) As this brightness round her limb diminishes rather suddenly, it resembles a narrow luminous border, and therefore does not seem to be the result of any optical deception. The light seems to decrease gradually between this border and the boundary between the illuminated and obscure parts of her disc. Schroëter had observed that the light appears strongest at the outward limb, from whence it decreases gradually, and in a regular progression towards the interior edge; but he differs from Herschel with regard to the sudden dimi
nution of this marginal light. “ With regard to the cause of this appearance,” says this celebrated astronomer,
may venture to ascribe it to the atmosphere of Venus, which like our own, is probably replete with matter that reflects and refracts light copiously in all directions. Therefore, on the border where we have an oblique view of it, there will, of consequence, be an increase of this luminous appearance.” Herschel considers the real surface of Venus to be less luminous than her atmosphere, and this accounts for the small number of spots which appear upon her disc. “For this planet” says he,“ having a dense atmosphere, its real surface will commonly be enveloped by it, so as not to present us with any variety of appearances. This also points out the reason why the spots, when any such there are, appear generally of a darker colour than the rest of the body.”
Schroëter seems to have been very successful in his observations upon Venus ; its atmosphere he considers to be very dense, not merely from the changes which take place in her dark spots, but from the illumination of her cusps, when she is near her inferior conjunction, when the enlightened ends of the horns reach far beyond a semi-circle. This astronomer also discovered several mountains in this planet, and found like those of Mercury and the moon, they were always highest in the southern hemisphere, their perpendicular heights being nearly as the diameters of their respective planets. He bas observed the southern horn much blunted, with an enlightened mountain in the dark hemisphere nearly 22 miles in altitude. (See Plate, Fig. 2.)
The luminous margin, already referred to, induced