There is a general consensus of opinion that great irregularities exist on the surface of Venus. These are made specially manifest to us in connection with the terminator or visible edge of the planet seen as an illuminated crescent. If the planet had a smooth surface this line would at all times be a perfect and continuous curve, instead of which it is frequently to be noticed as a jagged or broken line. Observations to this effect go back as far as 1643, when Fontana at Naples observed this to be the condition of the terminator. La Hire, Schröter, Mädler, Di Vico and many others down to the present epoch have noted the same thing. The fact that the southern horn of Venus is constantly to be seen blunted is so well established as to admit of no doubt, and this blunting is commonly ascribed to the existence of a lofty mountain, to which Schröter ascribed a height of 27 miles. Whatever we may think as to the precise accuracy of this figure, it seems impossible to doubt the main fact on which it depends; whilst a Belgian observer, Van Ertborn, in 1876 repeatedly saw a point of light in this locality which he regarded as due to Sun-light impinging on a detached peak, adjacent valleys remaining in shadow. This effect is common enough in the case of the Moon, and is familiar to all who are in the habit of studying the Moon. [Illustration: Fig. 9.—Venus, Dec. 23, 1885.] The existence on Venus of an atmosphere of considerable density and extent is well established. Proof of this is to be found in the marked diminution of the planet’s brilliancy towards the terminator; and in the faint curved line of light which occasionally may be seen when the planet is near inferior conjunction. When so situated, so much of the planet itself as can be seen illuminated shows as a narrow radiant crescent of light, ending off in two points called indifferently cusps or horns. It sometimes happens, however, that from the point of each cusp there runs round to the other cusp a faint continuation of the crescent, resulting in the general appearance of the planet being that of a nearly uniform ring of light. There is no known way in which the Sun can illuminate so much more than the half of Venus so as to permit of a perfect circle being visible except by supposing that an atmosphere exists on the planet and refracts (or transmits by bending, as it were, round the corner) a sufficient amount of Sun-light to give rise to the appearance in question. Further proof of the existence of an atmosphere on Venus is obtainable on those very rare occasions when the planet is seen passing across the disc of the Sun—a phenomenon known as a “Transit of Venus.” It then nearly always happens that a hazy nebulous ring of feeble light may be detected encompassing the planet’s disc indicative of course of the fact that the Sun’s rays are there slightly obstructed in reaching the eye of an observer on the Earth. Some observers