MOUNT WILSON SOLAR OBSERVATORY. 235 



machines occupy the remainder of the space on the west side of the 

 building. With the completion of the transfer of the photographic 

 negatives from the plate vault in the Hooker building to the new office 

 building, this room will be left available for the storage of optical disks, 

 grinding and polishing tools, and general supplies of special value. 



The strong need for well-equipped dark-rooms and enlarging rooms 

 in which to carry on the increasing amount of photographic work 

 required by the Observatory has led us to remodel for this purpose 

 the rooms in the rear of the main shop-building, used as computing 

 offices previous to the completion of the new office-building. Provi- 

 sion has been made for an office, two developing-rooms, an enlarging- 

 room equipped with a Cooper-Hewitt arc lamp and projection 

 apparatus for negatives up to 20 by 24 inches, a room in which is 

 installed an instrument for enlarging and widening spectra, a room 

 for chemical work and for washing negatives and photographic 

 prints, a dark-room mainly used for sensitizing plates, and a room 

 containing an electric plate-drier. A large amount of cupboard 

 space is available for the storage of plates and chemicals. 



In addition to these larger changes a number of minor alterations 

 have been made. These have included remodeling two of the small 

 offices at the front of the main shop-building in such a way as to pro- 

 vide office room for one of the laboratory observers; the construction 

 of a concrete driveway between the shop and the Hooker building; 

 and the erection of a small room above the photographic rooms for 

 the making of blue-prints. The electric wiring system of the entire 

 shop-building has also been modified and all exposed wires have been 

 placed in conduits. 



WORK OF THE INSTRUMENT SHOP. 



The most important work of the year has been the construction 

 of the large ruling-machine, under the supervision of Dr. Anderson, 

 of the Johns Hopkins University. As explained in the last report, 

 the machine was designed by Dr. Anderson, assisted by Mr. Pease, 

 and embodies the general principles of Rowland's successful ruling- 

 machines. Great care was taken in the selection of suitable metal 

 for the castings, which were obtained from Brown & Sharpe, and 

 for the screw and other parts of the machine. The extremely deli- 

 cate and difficult operation of cutting the screw was carried out by 

 Mr. Jacomini with great success, on the new Pratt & Whitney lathe, 

 after the ways had been scraped, with the high degree of accuracy 

 necessary for work of this nature. The method employed by Mr. 

 Jacomini in cutting the screw greatly reduced the magnitude of the 

 periodic errors below that usually encountered, so that the work 

 of grinding was much facilitated. In grinding the screw Dr. Ander- 

 son employed Rowland's well-known method, but introduced impor- 



