We have evidence just as direct that there is no atmosphere. This is very strikingly shown when the Moon, in its monthly progress among the stars, passes before one of them and occults it. Such an occultation is instantaneous, and is particularly impressive when either a disappearance or a reappearance occurs at the defective limb; that is to say, at the limb which is not illuminated by the Sun, and is therefore invisible. The observer may have a bright star in the field of view, showing steadily in a cloudless sky; there is not a hint of a weakening in its light; suddenly it is gone. The first experience of such an observation is most disconcerting; it is hardly less disconcerting to observe the reappearance at the dark limb. One moment the field of view of the telescope is empty; the next, without any sort of dawning, a bright star is shining steadily in the void, and it almost seems to the observer as if an explosion had taken place. If the Moon had an atmosphere extending upwards from its surface in all directions and of any appreciable density, an occultation would not be so exceedingly abrupt; and, in particular, if the occultation were watched through a spectroscope, then, at the disappearance, the spectrum of the star would not vanish as a whole, but the red end would go first, and the rest of the spectrum would be swept out of sight successively, from orange to the violet. This does not happen; the whole spectrum goes out together, and it is clear that no appreciable atmosphere can exist on the Moon. In actual observation so inappreciable is it that its density at the Moon's surface is variously estimated as 1/300th of that of the Earth by Neison, and as 1/10000th by W. H. Pickering. If the Moon possessed an atmosphere bearing the same proportion to her total mass as we find in the case of the Earth, she would have a density of one-fortieth of our atmosphere at the sea level.