878 CONDITION OF THE MOON ’S SURFACE.

from him; at one time tilting the floor eastwards,at another westwards, and at intermediate periods giv-ing every intermediate variety of tilt; these changes,moreover, having their maximum in turn at all epochsof the lunation. Combining this consideration withthe circumstance that very slight variations in thepresentation of a flattish surface will cause certainportions to appear relatively dark or relatively light,it appears to me that a case has not yet been madeout for those selenographical changes by which Mr.Birt has proposed to interpret these phenomena.

Nevertheless it cannot be insisted on too stronglythat it is from the detailed examination of the moon’ssurface that we can now alone hope for exact informa-tion as to its present condition and past history. Iwould even urge, indeed, that the detailed examina-tion at present being carried out is not sufficientlyexact in method. I should be glad to hear of suchprocesses of examination as were applied by Mr.Dawes to the solar spots. In particular it seems tome most important that the physiological effectswhich render ordinary telescopic observation andordinary eye-estimates of size, brightness, and colourdeceptive, should be as far as possible eliminated.This might be done by so arranging the observationsthat the conditions under which each part of the moonshould be studied might be as far as possible equalizedduring the whole progress of the lunation. Thus,returning to the case of the floor of Plato : this regionshould not be examined when Plato is near the ter-