first sight, it would seem as if the Moon might help us; for the Moon’s rim is similarly ringed by a lune of light. In her case the effect has been attributed to mountain slopes holding the Sun’s light at angles beyond the possibility of plains. But Mars has few mountains worthy the name. His terminator—that is, the part of the disk which is just passing in or out of sunlight, and discloses mountains by the way in which they catch the coming light before the plains at their feet are illuminated—shows irregularities quite inferior to the lunar ones, proving that his elevations and depressions are relatively insignificant.
Not due, then, to either mountains or mist, there is something we know that would produce the effect we see,—dust or water particles in the Martian air; that is, just as the Earth’s atmosphere is somewhat of a veil, so is the Martian one, and this veiling effect, though practically imperceptible in the centre of the disk, becomes noticeable as we pass from the centre to the edge, owing to the greater thickness of the stratum through which we look. At thirty degrees from the edge, our line of sight pierces twice as much of it as when we look plumb down upon the centre of the disk, and more yet as we approach the edge itself; in consequence, what would be diaphanous at the centre might well seem opaque toward the
