[Illustration: Fig. 15. Photograph of the moon made on September 15, 1919, with the 100-inch Hooker telescope (Pease). The ring-like formations are the so-called craters, most of them far larger than anything similar on the earth. That in the lower left corner with an isolated mountain in the centre is Albategnius, sixty-four miles in diameter. Peaks in the ring rise to a height of fifteen thousand feet above the central plain. Note the long sunset shadows cast by the mountains on the left. The level region below on the right is an extensive plain, the Mare Nubium.] [Illustration: Fig. 16. Photograph of the moon made on September 15, 1919, with the 100-inch Hooker telescope (Pease). The mountains above and to the left are the lunar Apennines; those on the left just below the centre are the Alps. Both ranges include peaks from fifteen thousand to twenty thousand feet in height. In the upper right corner is Copernicus, about fifty miles in diameter. The largest of the conspicuous group of three just below the Apennines is Archimedes and at the lower end of the Alps is Plato. Note the long sunset shadows cast by the isolated peaks on the left. The central portion of the picture is a vast plain, the Mare Imbrium.] The sharpness of the images given by the new telescope may be illustrated by some recent photographs of the moon, obtained with an equivalent focal length of 134 feet. In Fig. 15 is shown a rugged region of the moon, containing many ring-like mountains or craters. Fig. 16 shows the great arc of the lunar Apennines (above) and the Alps (below), to the left of the broad plain of the Mare Imbrium. The starlike points along the moon's terminator, which separates the dark area from the region upon which the sun (on the right) shines, are the mountain peaks, about to disappear at sunset. The long shadows cast by the mountains just within the illuminated area are plainly seen. Some of the peaks of the lunar Apennines attain a height of 20,000 feet. In less powerful telescopes the stars at the centre of the great globular clusters are so closely crowded together that they cannot be studied separately with the spectrograph. Moreover, most of them are much too faint for examination with this instrument. At the 134-foot focus the 100-inch telescope gives a large-scale image of such clusters, and permits the spectra of stars as faint as the fifteenth magnitude to be separately photographed. [Illustration: Fig. 17. Hubble's Variable Nebula. One of the few nebulae known to vary in brightness and form. Photographed with the 100-inch telescope (Hubble).] CLOSE DOUBLE STARS