208 CARNEGIE INSTITUTION OF WASHINGTON. 



measurements; the new method of measuring the velocity of Hght; the method 

 of detecting invisible sun-spots; the method of measuring the quality of see- 

 ing; the use of the Fabry-Perot interferometer for work on the aurora; the 

 use of pyrex glass to reduce the heat distortion of coelostat mirrors; the design 

 of the 50-foot interferometer telescope, and the design of a 75-foot spectrograph 

 for a new physical laboratory in Pasadena. The latter instrument will differ 

 materially from the 75-foot spectrograph of the 150-foot tower telescope, as the 

 focal length of its collimator can be made 13, 30, 50, or 75 feet, while cameras of 

 any desired dimensions, from a portrait lens of 10 inches aperture and 45 inches 

 focal length to a two-lens system of 75 feet focal length, can be used with any 

 suitable collimator. 



It goes without saying that in all of this work the Division of Drafting and 

 Design, the machine and optical shops, and the Construction Division on 

 Mount Wilson remain indispensable. The latter has built a reservoir on 

 Mount Wilson of 530,000 gallons capacity, to protect us against possible 

 droughts, such as have been threatened in recent years. The shops have been 

 operated to their full capacity, and the long Ust of instruments and optical 

 surfaces produced testifies to their efiiciency (p. 253). 



STAFF. 



The Director has continued his solar investigations, and has devoted much 

 time to the design of a new physical laboratory and a 50-foot interferometer 

 telescope. At the end of May he was forced by illness to go abroad for a 

 year's rest, leaving the Observatory in the charge of Dr. Adams. 



Dr. Walter S. Adams, Assistant Director, has remained in charge of the 

 department of stellar spectroscopy. Professor Frederick H. Seares, superin- 

 tendent of the Computing Division and editor of the Observatory publications, 

 has been engaged in photometric and related investigations. Dr. Arthur S. 

 King, superintendent of the Physical Laboratory, has made further studies of 

 spectra in the electric furnace, with special reference to problems relating to 

 ionization. Dr. Charles E. St. John has been engaged in wave-length deter- 

 minations and in researches on the solar rotation and the spectrum of Venus. 

 Dr. J. A. Anderson has continued his examination of the spectra of explosive 

 discharges and has conducted further tests with Mr. Jacomini of the ruhng- 

 machine. Mr. Harold D. Babcock has made wave-length determinations 

 with interference apparatus and has appHed a small interferometer to the 

 study of the auroral spectrum. Mr. Francis G. Pease has continued the 

 measurement of the diameters of stars with the 20-foot interferometer in 

 addition to his work on instrument design. Dr. Paul W. Merrill has extended 

 his investigations on the spectra of long-period variable stars and other 

 special types of stellar spectra. Dr. Adriaan van Maanen has measured 

 parallaxes and proper motions and has studied the motions in spiral nebulae. 

 Professor Alfred H. Joy, secretary of the Observatory, has been engaged in 

 determinations of radial velocity and of spectroscopic parallaxes. Mr. 

 Ferdinand EUerman has continued solar observations and has remained in 

 charge of the general photographic work of the Observatory. Dr. Seth B. 

 Nicholson has carried on observations on the sun and Venus, and in con- 

 junction with Dr. Pettit has made measurements of the radiation of stars 

 with thermopiles. Dr. Gustav Stromberg has taken part in stellar spectro- 



