66 REPORTS ON INVESTIGATIONS AND PROJECTS. 



in Pasadena. Miss Louise Ware was appointed a computer on July i, 

 and Miss Ruth E. Smith joined the computing division in August. Pro- 

 fessor G. W. Ritchey has been in charge of all of the principal work of 

 construction during the year. 



Dr. E. F. Nichols, Professor of Experimental Physics in Columbia Uni- 

 versity, was engaged in special investigations on IMount Wilson during 

 the summer. 



The Smithsonian Expedition, which began its investigations on Mount 

 Wilson in the spring of 1905, has continued its work this year, under the 

 charge of Mr. Charles G. Abbot, Acting Director of the Smithsonian 

 Astrophysical Observatory. Mr. Abbot has been assisted by Mr. Leonard 

 R. Ingersoll, of the University of Wisconsin. 



The astronomical latitude and longitude of the Solar Observatory were 

 determined by Messrs. Edwin Smith and John E. McGrath, assistants of 

 the United States Coast and Geodetic Survey, in December and January. 



INVESTIGATIONS IN PROGRESS. 



The program of investigations undertaken last year, with the instru- 

 ments then available, has been continued and extended. It now com- 

 prises : (i) Daily photography of the sun with the photoheliograph ; (2) 

 daily photography of the sun with the spectroheliograph ; (3) photog- 

 raphy of the spectra of sun-spots ; (4) photography of the spectra of the 

 flocculi, for determination of radial velocity; (5) spectrographic investiga- 

 tions of the solar rotation ; (6) holographic investigations of the solar 

 absorption ; (7) certain special studies of stellar spectra, with a grating 

 and prism spectrograph of high dispersion; (8) laboratory investigations; 

 (9) studies of the correlation of solar and magnetic phenomena, made 

 with the aid of a recording variometer. 



The Snow telescope has continued to give excellent results, though the 

 rapid deterioration of the mirror surfaces when constantly exposed to sun- 

 light requires that they be resilvered very frequently. When the mirrors 

 are even slightly tarnished, the change in focal length, during exposure 

 to the sun, is rapid. For this reason they are kept covered except when 

 the actual observations with the spectroheliograph, or with other instru- 

 ments, are in progress, and between exposures a strong draft of air is 

 blown upon them by electric fans. The use of an arc lamp, for setting the 

 slit of the spectroheliograph on the H line, materially reduces the time of 

 exposure of the mirrors to sunlight in this work. Experiments made in 

 the Pasadena laboratory have shown that the form of a mirror, after dis- 

 tortion produced by heating the front surface with an electric heater, 

 placed about a foot in front of the mirror, can be restored by heating the 

 rear surface with a similar electric heater. The figure of the mirror when 

 restored in this way is not perfect, but the advantages of the method have 



