“Very well, let us go on. The lunar Caucasus is the broader, but shorter, range of mountains at the northeastern corner of the _Mare Serenitatis_, and the Alps extend eastward from the Caucasus to a conspicuous dark oval close to the terminator, which is one of the most remarkable formations on the moon, and which, when we come to study it in one of the larger photographs, will probably interest you deeply because it is one of the places where recent studies have discovered indications of what may possibly be some form of lunar life. I wish now to direct your attention to the central and upper parts of the photograph. Running downward from the south, a little west of the terminator, you will perceive a double row of immense rings and ring plains. They are not only remarkable individually, but quite as remarkable for their juxtaposition in two long ranges. Among them, in the westernmost row, are three or four whose names you may remember—Maurolycus, Stöfler, Aliacensis and Werner. Still larger ones are included in the eastern row, the largest of all being at the bottom. It is rather a hexagon than a circle. It is 115 miles in diameter, and the flat plain inside the bordering mountains contains about 9,000 square miles. By close inspection you will perceive a small crater mountain near the northwestern side. This immense walled plain is named Ptolemæus after a great astronomer of antiquity, the author of the Ptolemæic system, which treated the earth as the center of the universe. “Still more interesting are the things visible farther south. You cannot fail to remark a very beautiful ring, a perfect circle, brightly illuminated on the eastern side, and having a bright point symmetrically placed in the exact center. It is named Tycho, after another great astronomer, and is generally regarded as the most perfect crater ring on the moon. It is 54 miles in diameter, and its walls are about 17,000 feet high on the inner side, more than a thousand feet higher than Mt. Blanc, the giant of the terrestrial Alps. Its central mountain is 5,000 feet high. The most remarkable thing about Tycho is the vast system of ‘rays’ or bands which seem to shoot out from it in all directions, traversing the surface of the moon, north, south, east, and west for hundreds of miles, and never turning aside on account of any obstacle. They lie straight across mountains, valleys, and plains. We have already seen one of them, the largest of all perhaps, crossing the _Mare Serenitatis_ and the _Lacus Somniorum_, in the northern hemisphere of the moon. Nobody knows exactly what these rays mean or what they consist of. We shall from this time on see them in all the photographs that we examine, and later I shall have more to say about them, and the speculations to which they have given rise.