PRESIDENTIAL ADDRESS SECTION A. 43 



In 1874 Mr. James Lick gave 700,000 dollars to provide a 

 telescope superior to and more powerful than any telescope yet 

 made and also a suitable observatory connected therewith. The 

 regents of the University of California are the trustees of this 

 bequest. In 1892 Mr. Charles T. Yerkes, of Chicago, offered to 

 purchase a pair of discs of optical glass, 42 inches in diameter, for 

 the University of Chicago, and to bear the entire cost of building 

 an equatorial refractor of 40 inches diameter and housing it in a 

 suitable observatory. 



Five years later saw the dedication of the Yerkes Observatory, 

 75 miles from Chicago, on Williams Bay, Lake Geneva. The tele- 

 scope is still the largest refractor engaged in astronomical research. 

 Ample preparation for advanced studies in theoretical and prac- 

 tical astronomy and in astrophysics is afforded by the courses 

 at the University of Chicago. After completing the necessary 

 preliminary work at Chicago, students who desire to devote 

 special attention to observational astronomy or to astro- 

 physics are adrnittecl to the Yerkes Observatory, where they are 

 given every possible facility, and as soon as they have had sufficient 

 preliminary training they are encouraged to undertake original 

 investigations of their own. Similar facilities are given at Berkeley 

 in connection with the Lick Observatory. The great need of the 

 astronomer is more and more light, and although the 40-inch 

 refractor marked a high level in light-gathering power, it was not 

 sufficient to meet the demand for more. A refractor, too, suffers 

 from the great disadvantage of not being able to bring all the 

 light from a star to one focus. Visitors to a big telescope are 

 much impressed by the violet halo which they see round a bright 

 star — a halo which the astronomer mentally ignores — caused by 

 the out-of-focus violet image. In a photographic refractor the 

 halo is red, centred with green, when the focus is adjusted for the 

 most active photographic rays. These differences in focus for dif- 

 ferent parts of the spectrum are inseparable from the use of a 

 refractor and are of great disadvantage in work on stellar spectra. 

 The reflector, on the other hand, reflects all colours impartially 

 and brings all to one common focus on the slit of the spectroscope 

 or on the photographic plate. 



To meet the need of a big reflector, Mr. John D. Hooker, of 

 Los Angeles, California, in 1906 gave 45,000 dollars for the pur- 

 chase of a glass disc 100 inches in diameter to be made into a 

 reflector for the Mount Wilson Observatory. The making of such 

 a disc of glass free from flaws is a very difficult matter, and the 

 first disc made was considered so faulty that it was rejected at 

 sight. So many difficulties arose, ending in failure to produce a 

 better one, that the first disc was ground to shape, polished and 

 tried, and the result was quite successful. 



The mounting of such a large mirror and the transport to 

 Mount Wilson of all the necessary accessories to such a large tele- 

 scope was a serious engineering problem, but the combined thought 

 and skill of many men resulted after eleven years of work in the 

 successful accomplishment in 1917 of the great project. In 



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