MOUNT WILSON SOLAR OBSERVATORY. 179 



photographic photometry with the 6o-inch reflector. About i6o photographs 

 have been measured by her, most of which were taken for the determination 

 of the effect produced on photographic magnitudes by small changes in 

 focus, and by the various conditions which affect the development of the 

 photographic plate. 



Miss Mary Wanda Sheldon was appointed to the Computing Division 

 July I, 1911, and Miss Inez A. Ensign August i, 1911. Miss Sheldon will 

 divide her time between the study of laboratory photographs and general 

 computations connected mainly with stellar photometry. Miss Ensign is 

 employing her entire time in the measurement and reduction of stellar 

 spectra. 



Miss Haines has remained in charge of the library as in the past. During 

 the year the cataloguing of the unbound volumes and pamphlets has been 

 completed. The accessions through purchase, since September i, 1910, 

 amount to y2 volumes, while, in addition, many valuable gifts have been 

 received. During the year 282 volumes have been bound. The total number 

 of bound volumes now in the library is 1,817. 



CONSTRUCTION DIVISION. 



The past year has been an active one as regards construction work, both in 

 the instrument-shop and on Mount Wilson. As in previous years, the gen- 

 eral supervision of the work has been in charge of the Director, with Mr. 

 Pease in immediate charge of the design of the instruments, Mr. Ayers of 

 the instrument-shop, and Mr. Jones of the construction work on Mount 

 Wilson. Mr. Ritchey has been in charge of the optical work upon the 100- 

 inch mirror, and Mr. Drew has had direct supervision of the design of the 

 lOO-inch telescope. 



The principal work carried on in the instrument-shop during the year has 

 been the large 75-foot spectrograph to be used with the 150- foot tower. The 

 instrument has now been completed with the exception of a few minor de- 

 tails, and was installed on Mount Wilson during the month of August. It is 

 designed for use either as a spectrograph or spectroheliograph, with a large 

 variety of dispersive units, all of which are directly interchangeable. Thus 

 the observer can by means of an electric motor bring into position beneath 

 the collimating lens a prism of small angle, a prism of large angle, a diffrac- 

 tion grating, or two gratings side by side, according to the work to be under- 

 taken. The instrument can be used at a focal length of either 30 or 75 feet, 

 a hoisting apparatus having been arranged to raise the entire base with the 

 various dispersive units to the 30-foot level. 



The work of the instrument-shop during the year has included, in addition 

 to the large spectrograph : 



The construction of parts of the small stellar spectrograph for use at the 

 primary focus of the 60-inch reflector, and of the quartz spectrograph. 



The partial rebuilding of the driving-clock of the 60-inch reflector and 

 various changes connected with it. 



