telescope and a power of 100. [Illustration: PLATE XVII. Apennines, Alps, and Caucasus. Paris Observatory.] 5. THE SYSTEMS OF BRIGHT RAYS, radiating from certain craters, remain the most enigmatic of the features of lunar scenery. Many of these systems have been traced and mapped, but we need only mention the three principal--those connected with Tycho, Copernicus, and Kepler, all shown on Plate XII. The Tycho system is by far the most noteworthy, and at once attracts the eye when even the smallest telescope is directed towards the full moon. The rays, which are of great brilliancy, appear to start, not exactly from the crater itself, but from a greyish area surrounding it, and they radiate in all directions over the surface, passing over, and almost completely masking in their course some of the largest of the lunar craters. Clavius, for example, and Maginus (Plate XIV.), become at full almost unidentifiable from this cause, though Neison's statement that 'not the slightest trace of these great walled-plains, with their extremely lofty and massive walls, can be detected in full,' is certainly exaggerated. The rays are not well seen save under a high sun--_i.e._, at or near full, though some of them can still be faintly traced under oblique illumination. In ordinary telescopes, and to most eyes, the Tycho rays appear to run on uninterruptedly for enormous distances, one of them traversing almost the whole breadth of the moon in a north-westerly direction, and crossing the Mare Serenitatis, on whose dark background it is conspicuous. Professor W. H. Pickering, who has made a special study of the subject under very favourable conditions, maintains, however, that this appearance of great length is an illusion, and that the Tycho rays proper extend only for a short distance, being reinforced at intervals by fresh rays issuing from small craters on their track. The whole subject is one which requires careful study with the best optical means. None of the other ray-systems are at all comparable with that of Tycho, though those in connection with Copernicus and Kepler are very striking. As to the origin and nature of these strange features, little is known. There are almost as many theories as there are systems; but it cannot be said that any particular view has commanded anything like general acceptance. Nasmyth's well-known theory was that they represented cracks in the lunar surface, caused by internal pressure, through which lava had welled forth and spread to a considerable distance on either side of the original chasm. Pickering suggests that they may be caused by a deposit of white powder, pumice, perhaps, emitted by the craters from which the rays originate. Both ideas are ingenious, but both present grave difficulties, and neither has commended itself to any very great extent to observers, a remark which applies to all other attempts at explanation.