1. THE GREAT GREY PLAINS.--These are, of course, the most conspicuous features of the lunar surface. A number of them can be easily seen with the naked eye; and, so viewed, they unite with the brighter portions to form that resemblance to a human face--'the man in the moon'--with which everyone is familiar. A field-glass or small telescope brings out their boundaries with distinctness, and suggests a likeness to our own terrestrial oceans and seas. Hence the name Maria, which was applied to them by the earlier astronomers, whose telescopes were not of sufficient power to reveal more than their broader outlines. But a comparatively small aperture is sufficient to dispel the idea that these plains have any right to the title of 'seas.' The smoothness which at first suggests water proves to be only relative. They are smooth compared with the brighter regions of the moon, which are rugged beyond all terrestrial precedent; but they would probably be considered no smoother than the average of our own non-mountainous land surfaces. A 2 or 2-1/2-inch telescope will reveal the fact that they are dotted over with numerous irregularities, some of them very considerable. It is indeed not common to find a crater of the largest size associated with them; but, at the same time, craters which on our earth would be considered huge are by no means uncommon upon their surface, and every increase of telescopic power reveals a corresponding increase in the number of these objects (Plates XIII., XV., XVII.). [Illustration: PLATE XIII. The Moon, September 12, 1903. Paris Observatory.]