THE PLANET VENUS.

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form, as in the case of Venus ; one nearer, when gibbous, as inthe case of Mercury . *

We have said in our last lecture that Mercury does notappear so brilliant as we should expect that it would, if itslight were for the most part reflected from a dense canopy ofcloud. On the other hand, the brilliancy of Venus is so great,that we have little doubt that what we see is chiefly cloud, andnot the true surface of the planet. Ohacornac found its bright-ness to be ten times greater than that of the most luminousparts of the Moon ; which would assign to it a light-reflectingpower fully five times as great as that of those parts, since thesunlight received by it is not quite twice as intense. But thiscan hardly be the case ; inasmuch as the late Professor Zollnerfound the average reflective power of the Moon ’s surface to besuch as to send back about £th of the light incident upon it.The best investigations yet made indicate that the light-reflecting power of Venus (or, as it is sometimes technicallytermed, its albedo, or whiteness) is rather more than three timesas great as that just stated for the Lunar surface as a whole.

The period of the rotation of Venus upon its axis rests uponstronger evidence than that of Mercury . So long ago as1666 the eldest Cassini, while observing it, in the clear sky ofItaly , watched a bright spot upon its surface, which returned toa similar position in about 23 hours. This observation wasafterwards, to some extent, confirmed by others made byBianchini, who about a.d. 1724 detected the apparent rotationof several dark spots.* Again, in 1789, Schroter, having-undertaken a careful examination of the disc with a seven-foot reflector, discovered a small luminous point in the darkhemisphere, slightly separated from the southern horn of thecrescent. This point he supposed to be a high mountain, whosesummit caught the Sun’s rays before they reached the sur-rounding regions. He also tells us, that a blunted appearance

* Bianchini announced the period as 24 days 8 hours, but Cassini’s son(who is generally known as Cassini II., or James Cassini) showed thatBianchini had not been able to watch the planet continuously, and thata rotation of 23 hours 20 minutes would equally satisfy his observations,by supposing that it had really gone round 25 times, while he imaginedthat it had only rotated once.