But though part of her lustre is due to her position, a part is her own. Direct visual observation, as we remarked above, shows her intrinsic brightness to be more than five times that of Mercury, square mile to square mile of surface for the two. Now this has been determined very carefully photometrically by Müller at Potsdam. The result of his inquiry was to indicate that Mercury shines with 0.17 of absolute reflection, Venus with 0.92. So high a value has seemed to many astronomers impossible, because so far surpassing that which has tacitly been taken as the _ne plus ultra_ of planetary brightness, that of cloud, 0.72. Now, one of the direct outcomes of the study of Venus at the Lowell Observatory was an explanation of this seemingly incredible phenomenon. When the planet came to be critically examined there under conditions of seeing which permitted discovery, markings very faint, but nevertheless assurable, stood presented on the planet’s face. These markings, of which we shall have more to say in a moment, had this of pertinency to our present point, that they kept an invariable position to one another. They thus betrayed themselves to be surface features. Furthermore, their dimness was as invariable an attribute of them as their place. They were not obscured on some occasions and revealed at others, but stayed, so far as one might judge, permanently the same. They were thus neither clouds themselves nor subject to the caprice of cloud. The old idea that Venus was a cloud-wrapped planet and owed her splendor to this envelope, vanished literally into thin air. It is precisely because she is not cloud-covered that her lustre is so great. She “clothes herself with light as with a garment” by a physical process of some interest. As becomes the Mother of the Loves, this is gauze of the most attenuated character, and yet a wonderful heightener of effect. For it consists solely of the atmosphere that compasses her about. It is well known that a substance when comminuted reflects much more light than when condensed into a solid state. Now an atmosphere is itself such a comminuted affair, and, furthermore, holds in suspension a variety of dust. This would particularly be the case with the atmosphere of Venus, as we shall have reason to see when we consider the conditions upon that planet made evident by study of its surface markings. To her atmosphere, then, she owes four-fifths or more of her brilliancy. And this stands corroborated by the low albedo of both Mercury and the Moon, which have no atmosphere, and by the intermediate lustre of Mars, which has some, but little.[5] [5] _Astr. Nach._ No. 3406. Monthly Notices R. A. S., March, 1897.