MOUNT WILSON SOLAR OBSERVATORY. 263 



Several members of the Division, notably Miss Carolyn Burns, 

 Miss Davis, and Miss Richmond, have assisted with checking and proof- 

 reading. 



Mrs. Longacre also assisted with the editorial work and remained 

 in charge of the library until January. Her resignation took effect on 

 March 1, and Miss Connor was appointed to fill the vacancy on July 15. 

 The accessions of bound volumes number 365; 205 by purchase, 138 by 

 binding, and 22 by gift. The total number of volumes is now 4,256. 



CONSTRUCTION DIVISION. 



DRAFTING AND DESIGN. 



Mr. Pease has continued in charge of drafting and design, and has 

 paid much attention to the erection of the 100-inch mounting and 

 the inspection of parts made in outside shops. Mr. Nichols has acted 

 as chief draftsman since the first of January. About half of the year 

 has been spent on 100-inch telescope drawings and the remainder on 

 other work. 



Among the 100-inch telescope drawings completed are those of the 

 jib crane, shutter mechanism, observing platform wind-screen, optical 

 system for reading circles from main operating desk, right-ascension 

 direct setting circle, and temperature control for the mirror. Draw- 

 ings for other instruments include : 



Camera for 10-inch portrait lens, with double-slide plate-holder 

 (14 by 17 inches) and numerous other attachments. 



Alterations in the Snow telescope, including new spectrograph pit, 

 coelostat and concave-mirror slow motions, and 30-foot spec- 

 trograph with all its attachments. 



Short-focus camera for Cassegrain focus of 60-inch telescope. 



Adapter for registering photometer on 6-inch refractor. 



Additions to measuring machines. 



Concave-grating spectrograph of 3.5 m. focus. 



Many drawings have also been made for charts and illustrations. 



WORK OF THE OPTICAL SHOP. 



The work of parabolizing the 100-inch mirror has been brought by 

 Mr. Ritchey to successful completion. For many months, as the 

 figure was gradually brought nearer to a true paraboloid, daily optical 

 tests were made both at the center of curvature and at the primary 

 focus. The focal tests involved the use of the 60-inch plane mirror 

 which was made here for this purpose, and the close agreement and 

 consistency of the daily results secured by the two methods gave 

 much satisfaction and confidence in the outcome. 



In general, the tests at the center of curvature were found to be most 

 useful in determining the total amount of parabolization. In these, 

 successive zones of the surface are examined, one after another, from 



