The 60-foot tower on the extreme left, which is at the edge of a precipitous canon 1,500 feet deep, is the vertical telescope of the Smithsonian Astrophysical Observatory. Above it are the "Monastery" and other buildings used as quarters by the astronomers of the Mount Wilson Observatory while at work on the mountain. (The offices, computing-rooms, laboratories, and shops are in Pasadena.) Following the ridge, we come successively to the dome of the 10-inch photographic telescope, the power-house, laboratory, Snow horizontal telescope, 60-foot-tower telescope, and 150-foot-tower telescope, these last three used for the study of the sun. The dome of the 60-inch reflecting telescope is just below the 150-foot tower, while that of the 100-inch telescope is farther to the right. The altitude of Mount Wilson is about 5,900 feet.] The crystalline lens of the human eye, limited by the iris to a maximum opening about one-quarter of an inch in diameter, was the only collector of starlight available to the Greek and Arabian astronomers. Galileo's telescope, which in 1610 suddenly pushed out the boundaries of the known stellar universe and brought many thousands of stars into range, had a lens about 2-1/4 inches in diameter. The area of this lens, proportional to the square of its diameter, was about eighty-one times that of the pupil of the eye. This great increase in the amount of light collected should bring to view stars down to magnitude 10.5, of which nearly half a million are known to exist. It is not too much to say that Galileo's telescope revolutionized human thought. Turned to the moon, it revealed mountains, plains, and valleys, while the sun, previously supposed immaculate in its perfection, was seen to be blemished with dark spots changing from day to day. Jupiter, shown to be accompanied by four encircling satellites, afforded a picture in miniature of the solar system, and strongly supported the Copernican view of its organization, which was conclusively demonstrated by Galileo's discovery of the changing phases of Venus and the variation of its apparent diameter during its revolution about the sun. Galileo's proof of the Copernican theory marked the downfall of mediaevalism and established astronomy on a firm foundation. But while his telescope multiplied a hundredfold the number of visible stars, more than a century elapsed before the true possibilities of sidereal astronomy were perceived. [Illustration: Fig. 4. The 100-inch Hooker telescope.] STRUCTURE OF THE UNIVERSE Sir William Herschel was the first astronomer to make a serious attack upon the problem of the structure of the stellar universe. In his first memoir on the "Construction of the Heavens," read before the Royal Society in 1784, he wrote as follows: