Though unfavourably placed for observation, there is no lunar feature which can compare in grandeur with Clavius when viewed either at sunrise or sunset. At sunrise the great plain appears first as a huge bay of black shadow, so large as distinctly to blunt the southern horn of the moon to the naked eye. As the sun climbs higher, a few bright points appear within this bay of darkness--the summits of the walls of the larger craters--these bright islands gradually forming fine rings of light in the shadow which still covers the floor of the great plain. In the East some star-like points mark where the peaks of the eastern wall are beginning to catch the dawn. Then delicate streaks of light begin to stream across the floor, and the dark mass of shadow divides itself into long pointed shafts, which stretch across the plain like the spires of some great cathedral. The whole spectacle is so magnificent and strange that no words can do justice to it; and once seen it will not readily be forgotten. Even a small telescope will enable the student to detect and draw the more important features of this great formation; and for those whose instruments are more powerful there is practically no limit to the work that may be done on Clavius, which has never been studied with the minuteness that so great and interesting an object deserves. (Clavius is No. 13, Plate XIX. See also Plates XIII. and XV., and Fig. 22, the latter a rough sketch with a 2-5/8-inch refractor.) From such gigantic forms as these, the craters range downwards in an unbroken sequence through striking objects such as Tycho and the grand Copernicus, both distinguished for their systems of bright rays, as well as for their massive and regular ramparts, to tiny pits of black shadow, a few hundred feet across, and with no visible walls, which tax the powers of the very finest instruments. Schmidt's great map lays down nearly 33,000 craters, and it is quite certain that these are not nearly all which can be seen even with a moderate-sized telescope. [Illustration: PLATE XV. Clavius, Tycho, and Mare Nubium. Yerkes Observatory.] As to the cause which has resulted in this multitude of circular forms, there is no definite consensus of opinion. Volcanic action is the agency generally invoked; but, even allowing for the diminished force of gravity upon the moon, it is difficult to conceive of volcanic action of such intensity as to have produced some of the great walled-plains. Indeed, Neison remarks that such formations are much more akin to the smaller Maria, and bear but little resemblance to true products of volcanic action. But it seems difficult to tell where a division is to be made, with any pretence to accuracy, between such forms as might certainly be thus produced and those next above them in size. The various classes of formation shade one into the other by almost imperceptible degrees. [Illustration: FIG. 22. CLAVIUS, June 7, 1889, 10 p.m., 2-5/8 inch.]