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or shortly after sunrise, if it rises before the sun. Supposing it east of the sun, it will probably be visible in the telescope at any time after noon, but the air is generally disturbed by the sun's rays so that it is hardly possible to make a good observation at that time. Late in the afternoon the air grows steadier, so that the planet can be better observed. But, after sunset, the planet is seen through a continually increasing extent of atmosphere, so that the seeming disturbance again begins to increase. Owing to these circumstances it is the most difficult of all the planets to study in a satisfactory way, and observers differ very much as to what can be seen on its surface.

The first observer who thought he could see any features on the surface of this planet was Schröter, a German. When Mercury presented the form of a crescent he fancied that its south horn seemed blunted at intervals. He attributed this to the shadow of a lofty mountain; and by observing the intervals between the blunted appearance he concluded that the planet revolved on its axis in twenty-four hours and five minutes. But Sir William Herschel, who observed at the same time with much more powerful instruments, could not see anything of the kind.

Until quite recently nearly all observers agreed with Herschel that no time of rotation could be certainly determined. But a few years since, Schiaparelli, observing with a fine telescope in the beautiful sky of northern Italy, noticed that the aspect of the planet seemed unchanged day after day. He was thus led to the conclu-