scrutinising Venus when in transit have thought that they were able to obtain, by means of the spectroscope, traces of aqueous vapour on the planet, but the evidence of this does not appear to be altogether clear or conclusive. [Illustration: Fig. 10.—Venus near conjunction as a thin crescent, Sept. 21, 1887 (Flammarion).] Everybody may be presumed to be acquainted with the spectacle popularly known as “The Old Moon in the New Moon’s Arms” whereby when the Moon is only about two or three days old and exhibits but a narrow crescent of bright light, yet the whole outline of the disc is traceable on the sky. A phenomenon analogous to this may often be seen in the case of Venus when near its inferior conjunction. With the Moon the cause is due to the reflection of Earth-light (so to speak) to the Moon, but that explanation seems inadequate in respect of Venus, because it is conceived that the amount of Earth-light available is altogether insufficient for the purpose. Many other explanations have been put forward including phosphorescence on the surface of Venus, electrical displays in the nature of terrestrial auroræ, and what not, but it must be frankly confessed that astronomers are all at sea on the subject. The existence of snow at the poles of Venus has been suspected by observers of tried skill and experience such as Phillips and Webb, though the idea was first broached by Gruithuisen in 1813. Flammarion’s observations during 1892 and the two following years are distinctly confirmatory of this idea. He adds that as both polar caps are visible at the same time the planet’s axis cannot be much inclined to the plane of its orbit. Compared with all the other planets the absolute brightness of Venus stands very high. Of course it must be understood that by this phrase “absolute brightness” no more is meant than its reflective power. Venus is what it is by virtue of its power of reflecting Sun-light; presumably it has no inherent brightness of its own. What its reflective power is was probably never more effectively brought under the notice of a human eye than on September 26, 1878, when Nasmyth enjoyed an opportunity of seeing Venus and Mercury side by side for several hours in the same field of view. He speaks of Venus as resembling clean silver and Mercury as nothing better than lead or zinc. Seeing that owing to its greater proximity to the Sun the light incident on Mercury must be some 3½ times as strong as the light incident on Venus, it follows that the reflective power of Venus must be very great. As a matter of fact it has been calculated to be nearly equal to newly fallen snow; in other words to reflect fully 70 per cent. of the light which impinges on it.