362 ANNUAL REPORT SMITHSONIAN INSTITUTION. 1910. 



necessary correcting movements, thus avoiding the useless waste of 

 exertion in the older method. Finally, a very small, convex, hy- 

 perbolic mirror, placed near the principal focus, allows them to give 

 to the telescope the Cassegrainian form, increasing its focal length 

 for the stud}' of the brighter stars. 



With this instrument, and indeed, with its old form of mounting, 

 the Lick observers were able to obtain a superb series of plates of 

 the nebulae, which are rivalled only by those obtained during the 

 past year at Mount Wilson. They hope to photograph thus 120,000 

 nebulas, possibly many more, and our present catalogues contain only 

 13,000. 



The spiral form is much more general among nebulas than was 

 formerly supposed, a result which becomes even more interesting 

 since the Crossley reflector has enabled Fath to obtain the spectra of 

 several of them. He found their spectra very remarkable and com- 

 posed of three distinct types; a continuous spectrum, bright nebular 

 lines, and dark absorption lines. The theory that these nebulae are 

 very condensed masses of stars is therefore supported by this. 



THE YERKES OBSERVATORY. 



Less favored than the Lick Observatory by its climate, the Yerkes 

 Observatory is near Chicago; it was dedicated in 1897 in the pres- 

 ence of a gathering of astronomers from the whole world. It owes 

 its inception to the energetic initiative, assisted by the means of a 

 wealthy manufacturer, of Dr. Hale, who was its first director and 

 who proposed to make it a center of the first rank. Its great tele- 

 scope, with an objective (pi. 3) of 1.02 meters (40 inches), derived 

 much profit in its construction from the one previously built for the 

 Lick Observatory. All perfections possible seem to be combined in 

 this instrument, which, with its moving dome, cost some $170,000. 

 The moving floor is raised by electrical means, the clock rewinds 

 itself, and yet other motors direct the telescope to the desired place — 

 indeed, no convenience has been omitted. 



One sees here vividly how American methods of construction dif- 

 fer from ours, how little they concern themselves with customs so 

 rigorously observed elsewhere. Our great instruments always seem 

 to be built according to some former shop rules, very different from 

 the practice in the construction of the machinery of ordinary manu- 

 factories. Such is not the case with the great American telescopes; 

 their parts are more massive, less fragile, and have the appearance 

 of the machinery in the ordinary commercial industries. Everything 

 must conduce to regularity of operation rather than to surface re- 

 finements. Precision becomes illusory where there is flexure and the 

 strains tend constantly to destroy the perfection of the surfaces and 

 their adjustments. 



