170 REPORTS ON INVEISTIGATIONS AND PROJECTS. 



The Observatory secured in July of this year a 6-iiich plane grating made 

 by Michelson, of the University of Chicago, which has been placed in the 

 30-foot tower spectrograph. The spectra obtained with this instrument are 

 of such a high degree of excellence that it has seemed desirable to attempt 

 with it, to some extent at least, the determination of the wave-lengths of the 

 tertiary standards in the solar spectrum upon the international system. In 

 addition to these wave-lengths the investigation will provide the values of 

 the displacements of many solar lines due to the pressure of the sun's re- 

 versing layer. 



A number of photographs of the flash spectrum have been obtained with 

 this instrument by Mr. Ellerman, 



THE 60-INCH REFLECTOR. 



The principal addition to the equipment of the 60-inch reflector during the 

 year has been the large three-prism spectrograph employed with the Casse- 

 grain combination of mirrors at an equivalent focal length of 80 feet. This 

 instrument was built by Gaertner, of Chicago, in accordance with designs 

 made at the Observatory, and was installed on Mount Wilson in December 

 of last year. The construction of the spectrograph is such as to admit of a 

 wide variety of modifications, according to the brightness and character of 

 the object to be photographed. The number of prisms may be varied from 

 three to one, cameras with a focal length up to 40 inches may be employed, 

 and the positions of the prisms may be varied to bring into minimum devia- 

 tion any part of the visible spectrum. The instrument is of rigid construc- 

 tion, and is very well adapted for determinations of radial velocity, having 

 an automatic temperature control and a comparison-spectrum apparatus pro- 

 viding for the use of the electric arc. 



A number of changes and modifications have been made during the year 

 in connection with the telescope itself. The principal one of these was the 

 addition of a gear-train to the driving-clock in order to increase its power. 

 It was found during the autumn of 1910 that the power was insufficient to 

 drive the telescope satisfactorily in all positions with the considerable varia- 

 tions of weight upon the bearings involved in the use of different accessory 

 instruments. By the addition of a gear-train to the driving-clock its power 

 has been increased about 50 per cent and the difficulty has been removed 

 completely. In addition, the base upon which the clock rests has been 

 strengthened and made more rigid. Other minor alterations include the 

 insertion of ball bearings in the slow-motion motors for moving the telescope 

 in right ascension and declination, and strengthening of the fastenings of the 

 cover of the large cast-iron flotation tank to prevent mercury leaks. A steel 

 ladder curved to the form of the dome has been fastened near the shutter 

 opening, in order to facilitate reaching the observing platform. 



The Observatory now has under construction by the Union Iron Works 

 of San Francisco an additional steel frame, or "cage," to be placed upon the 

 end of the tube of the reflector in order to provide for work directly in the 



