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should have done had it been caused by regular refraction, but commenced at a point quite near one end of the arc. This appearance was explained by Russell, of Princeton, who showed that the atmosphere is so full of vapour that we cannot see the light of the sun by direct refraction through it. What we see is an illuminated stratum of clouds or vapour floating in an atmosphere. Such being the case, it is not at all likely that astronomers on the earth can ever see the solid body of the planet through these clouds. Hence the supposed spots could only have been temporary clouds, continually changing.

To illustrate the illusions to which the sight of even good observers may be subject, we may mention the fact that several such observers have supposed the whole hemisphere of Venus to be visible when the planet was near inferior conjunction. It then had the appearance familiarly known as "the new moon in the old moon's arms," with which everyone who observes our satellite when a narrow crescent is familiar. In the case of the moon it is well known that we thus see the dark hemisphere by the light reflected from the earth. But in the case of Venus there is no possibility of a sufficient reflection of light from the earth, or any other body. The appearance has sometimes been explained by a possible phosphorescence covering the whole hemisphere of Venus. But it is more likely due to an optical illusion. It has generally been seen in the daytime, when the sky is brightly illuminated, and when any faint light like that of phosphorescence would be completely in-