_Surface-markings._—In 1666 and the following year J. D. Cassini observed several bright spots on Venus and also two obscure markings; but the latter were extremely faint and of irregular extent, so that little could be gleaned from them. He watched these forms closely and remarked certain changes in their positions, which finally enabled him to determine the period of the planet’s rotation. In 1726 and 1727 Bianchini, at Rome, repeatedly observed dark spots, and their outlines seem to have been so consistent that he depicted them on a chart and gave them names. But J. Cassini, at Paris, failed to confirm these results, though he used telescopes of 82-and 114-feet focus; and it was supposed the climate of Paris was not suitable for such delicate observations. Schröter reviewed this planet in 1788 and later years, and succeeded in detecting various markings and irregularities in the terminator and cusps. He announced that he had seen the S. horn of the crescent truncated, so that a bright point was apparently isolated at its extremity. From this he concluded there must be mountains of great altitude on the planet, and the perpendicular height of one of these he computed at 22 miles, which is four times the height of the most lofty mountain on the Earth. If the surface of Venus were uniformly level, then her cusps would taper gradually away to points, and no such deformation as that described by Schröter could possibly be produced. And there is strong negative evidence among modern observations as to the existence of abnormal features; so that the presence of very elevated mountains must be regarded as extremely doubtful, if, indeed, the theory has not to be entirely abandoned. The detached point at the S. horn shown in Schröter’s telescope was probably a false appearance due to atmospheric disturbances or instrumental defects. Whenever the seeing is indifferent, this planet assumes some treacherous features which are very apt to deceive the observer, especially if his telescope is faulty. Spurious details are seen, which quite disappear from the sharp images obtained in steadier air with a good glass. I have never observed truncation in either of the horns of Venus; but on certain occasions, when the planet has been ill-defined in passing vapours, it was most easy to believe that a fragment became detached from the extremity of the cusp, just in the manner described by Schröter. But close attention has showed the effect to be false, and revealed its cause. It was the rippling of the image that gave rise to the apparently dissevered cusp, in the same way that passing air-waves and resulting quivers in the image of Saturn’s ring will sometimes produce displacements, so that the observer momentarily sees several black divisions, and the edges are multiplied and superimposed one on another. Refraction, exercised by heated vapours in crossing objects, is obviously the source of all this.