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and Venus always present the same face to the sun, as the moon presents the same face to the earth. Schiaparelli reached this conclusion by noticing that a number of exceedingly faint spots could be seen on the southern hemisphere of Venus for several days in succession in the same position day after day. He could observe the planet through several hours on each day, and the constancy of the spots precluded the idea that the planet made one rotation and a little more in the course of a day. Lowell was led to the same conclusion by careful study of the planet at his Arizona observatory.

The latest conclusion has been reached by the spectroscope. We have already explained how, with this instrument, it can be determined whether a heavenly body is moving toward us or from us. The principle applies to a planet which we see by the reflected light of the sun as well as to a star. Hence, if Venus rotates, one part of its disk will be moving toward us, and the other from us. By comparing the dark lines of the spectrum shown by the two edges of the disk of Venus it can then be determined how various points of the disk are moving with respect to the earth. It was thus found by Belopolsky that the planet was affected by a quite rapid rotation. The observation is so difficult, and the displacement of the lines so small, that it was not possible to state a very certain result, although the general fact was made very probable. On the whole we must regard this conclusion as the most likely that has yet been reached, although it is at variance with the observations of Schiaparelli, as well as those of the Lowell Observatory. But the spectro-