lem in solar photography, which had long been the goal sought for by eminent astronomers, established for Hale at once an international reputation as an astrophysicist.

Soon after the establishment of Chicago University, Hale was appointed to the Faculty and put in charge of the research department of astronomy. The wisdom of putting so young a man in so

responsible a position, the sequel has proved. When, in the summer of 1891, it became known that the two 40-inch disks made for an observatory in Southern California were for sale, Hale made up his mind that the university must have them; and through his energy a gentleman was soon found who was willing, not only to pay for the disks and to have them ground and mounted, but also to build what is acknowledged to be the finest observatory in the world. This gentleman was Mr. Charles T. Yerkes, of Chicago, for whom the observatory is named. The site chosen was at Williams Bay on Lake Geneva, Wisconsin,- a beautiful lake about seventy-five miles from Chicago. The winter of 1894 was spent by Professor Hale and his wife in Berlin, where the plans of the new observatory were perfected with the greatest care.

The observatory was completed in the summer of 1897, and dedicated in the following October. Its imposing exterior is shown in Figure 2, and the interior of the great dome containing the 40inch telescope in Figure 3. The telescope itself is 62 feet in length, and the dome 90 feet in diameter. The floor, 75 feet in diameter, can be raised or lowered through a distance of 23 feet, the dome revolved, the shutter opened or closed, and the telescope pointed in any direction by the operator standing on the floor and manipulating the various rheostats seen on the floor at the right of the telescope mounting. As director of this large institution, Hale has shown no less executive ability combined with infinite tact than he had already shown ability as an investigator.

A unique feature of the Yerkes Observatory is its splendidly equipped optical and machine shop for the construction of the elaborate instruments which it is necessary to use either as accessories to the large telescope or for new work in hand. Practically, all of the superb spectrographs, spectroheliographs, colostats, and mirrors now in use at the observatory have been made in the observatory shops from original designs worked out by Professor Hale and his associates. Figure 4 is a view of his new spectroheliograph just completed, attached to the end of the 40-inch refractor, by means of which photographs similar to that shown in Figure 1 are taken, only on a very much larger scale, the image of the sun being seven

inches in diameter. The photographs of the prominences which this instrument will furnish during the next period of solar activity, about four years hence, will be on a scale of magnificence unprecedented. Figure 5 is a photograph (not enlarged) of a small sun spot and surrounding faculæ recently taken with this new instru

ment.

Another important research to which Hale has given much atten

tion is the photography of the corona without a total eclipse. This problem, as is well known, has long baffled the skill of astronomers; and, indeed, it remains unsolved at the present time. He first attacked this problem with Professor Keeler on Pike's Peak in 1893. During the spring of 1894 he and Mrs. Hale, together with Professor Ricco, ascended Mount Etna, where they lived for a number of days while, with a new method and apparatus, he again tried to photograph the corona. Failure, however, seemed only to serve to

increase his ingenuity; for he soon after proposed an entirely novel method, which is being worked upon now with every indication of ultimate success. This method consists in mapping the corona not by its light rays, but by the heat which it radiates to a bolometer or radiometer.

Hale's researches have not, however, been confined to solar investigations alone. Many other important spectroscopic investi

gations have also been worked out in the laboratory. With the great light-collecting power of the 40-inch telescope at his disposal, he has recently been working on the spectra of certain peculiar red stars, and has been able to draw some important conclusions relative to their classification in the system of stellar evolution. It is known that the spectra of all stars can be arranged in one of four groups, or "types," each type being characteristic of one stage in the evolution of the star. The spectra of certain stars, however,

seemed to offer exceptions; but Hale has shown from their spectra that they can be so arranged as to show a gradual transition from the accepted III type to the IV type, corresponding to their respective ages, or stages of evolution. Figure 6 is an enlarged photograph of the spectra of four of these stars of the IV type, showing

the gradual change in spectrum from one to the next, from which these far-reaching conclusions may be drawn.

The most recent contribution from the Yerkes Observatory is the demonstration that visual telescopes, like the Lick and Yerkes instruments, may be used with great success for photographic work. In visual instruments the objectives are corrected for achromatism. so as to bring to a sharp focus those rays most affecting the eye,