of the "terminator"--or dividing-line between
darkness and light in the planet's phases--first remarked by Schroeter,
and again clearly seen by Trouvelot in 1878 and 1881.[817] The
displacement, during four days, of certain brilliant and dusky spaces on
the disc indicated to Mr. Denning in 1882 rotation in about twenty-five
hours; while the general aspect of the planet reminded him of that of
Mars.[818] But the difficulties in the way of its observation are
enormously enhanced by its constant close attendance on the sun.

In his sustained study of the features of Mercury, Schroeter had no
imitator until Schiaparelli took up the task at Milan in 1882. His
observations were made in daylight. It was found that much more could be
seen, and higher magnifying powers used, high up in the sky near the
sun, than at low altitudes, through the agitated air of morning or
evening twilight. A notable discovery ensued.[819] Following the planet
hour by hour, instead of making necessarily brief inspections at
intervals of about a day, as previous observers had done, it was found
that the markings faintly visible remained sensibly fixed, hence, that
there was no rotation in a period at all comparable with that of the
earth. And after long and patient watching, the conclusion was at last
reached that Mercury turns on his axis in the same time needed to
complete a revolution in his orbit. One of his hemispheres, then, is
always averted from the sun, as one of the moon's hemispheres from the
earth, while the other never shifts from beneath his torrid rays. The
"librations," however, of Mercury are on a larger scale than those of
the moon, because he travels in a more eccentric path. The temporary
inequalities arising between his "even pacing" on an axis and his
alternately accelerated and retarded elliptical movement occasion, in
fact, an oscillation to and fro of the boundaries of light and darkness
on his globe over an arc of 47 deg. 22', in the course of his year of
88 days. Thus the regions of perpetual day and perpetual night are
separated by two segments, amounting to one-fourth of the entire
surface, where the sun rises and sets once in 88 days. Else there is no
variation from the intense glare on one side of the globe, and the
nocturnal blackness on the other.

To Schiaparelli's scrutiny, Mercury appeared as a "spotty globe,"
enveloped in a tolerably dense atmosphere. The brownish stripes and
streaks, discerned on his rose-tinged