MOUNT WILSON SOLAR OBSERVATORY. 175 



has given a large amount of time to computations connected with the investi- 

 gation of the solar rotation. 



Miss Wickham continued, up to the time of her resignation on February 

 I, her measures of laboratory photographs taken for the investigation of the 

 Zeeman effect. She also measured a considerable number of chromosphere 

 plates and one of the high-dispersion stellar spectrum negatives. 



Although the books and pamphlets of the library had been roughly classi- 

 fied, no attempt at cataloguing was made until the appointment of Miss 

 Haines on March i. Since then the classification has been completely revised 

 and all of the bound volumes have been catalogued. The loss of books as a 

 result of the burning of the quarters was serious, but we have succeeded in 

 replacing most of those burned. The accessions during the year through 

 purchase amount to about 125 volumes, while in addition the value of the 

 collection has been greatly enhanced through the gift of a large number of 

 volumes from various institutions and societies. At present the bound vol- 

 umes in the collection number 1,451. 



CONSTRUCTION DIVISION. 



All of the construction work of the year has been under the general super- 

 vision of the Director, with Mr. Ayers in immediate charge of the work in 

 the instrument shop and Mr. Jones in immediate charge of the work on 

 Mount Wilson. The difficult work involved in the details of the design of 

 the instruments built during the year has been under the immediate charge 

 of Mr. Pease. Mr. Ritchey has remained in charge of the design of the 

 lOO-inch telescope. 



Among the instruments constructed in the instrument shop during the past 

 year the most important are the following : 



The new double-slide plate-carrier for the 60-inch reflector. 



The coelostat, second mirror support, and lens mounting for the 150- foot 

 tower telescope, including tracks, and slow-motion and focusing attachments. 



The larger portions of the 75-foot spectrograph and spectroheliograph to 

 be used with the 150- foot tower telescope; also the slits, with their supports 

 and various auxiliary attachments. 



Additional parts of the loo-inch grinding-machine, which complete its 

 construction. 



In addition a large number of smaller pieces of apparatus have been built, 

 and minor repairs made on existing instruments. 



THE ONE HUNDRED AND FIFTY FOOT TOWER TELESCOPE. 

 The erection of the tower telescope was begun last year and two of the 

 lower sections were completed before the opening of the rainy season put 

 an end to the construction work. The large pit, 78 feet deep, beneath the 

 tower was also completed and thoroughly dried during the winter by arti- 

 ficial means. Erection work was begun again in March of this year and 

 continued until September. With the exception of the spectroscopic equip- 

 ment still under construction in the Pasadena instrument shop, the tower 



