MOUNT WILSON SOLAR OBSERVATORY. 211 



The instrument shop, which has been operated at its full capacity, has 

 constructed the following instruments and accessories during the year : 



Focal plane spectrograph, mounted on new cage, for the 6o-inch reflector. 



Concave grating spectrograph, with interferometer attachment, also for 

 the 6o-inch reflector (not yet completed). 



Completion of 75-foot spectrograph, including collimator and camera slits ; 

 polarizing apparatus ; guiding, focusing, and centering devices ; comparison 

 arc; mountings of lenses, prisms, gratings, and mirrors, and their installa- 

 tion; liquid prism (in part) ; hoisting mechanism; repairs to motors, etc. 



Completion of 150-foot tower telescope, including mountings and focus- 

 ing scales for objectives; water-jackets, tanks, and thermostat for controlling 

 the temperature of the mirrors, and their installation with circulating pump, 

 motor, and piping ; reduced speed of elevator, and minor repairs. 



Large measuring machine for plates 8 by 20 inches. 



Six small measuring machines (five unfinished). 



An 8 by 10 plate carrier for 60-inch reflector. 



Installation of a 500-volt electric generator with alternating-current driving 

 motor and a 17 kw. direct current generator with 25-horsepower alternating- 

 current driving motor, in addition to much minor work on instruments and 

 accessories, for the physical laboratory. 



Apparatus for the optical shop, including edge bands and supports for 

 testing the 100-inch mirror; testing frame and truck for 60-inch plane 

 mirror; loo-inch polishing tool, with supporting and driving device; 36-inch 

 polishing tool, with adapter for driving; knife-edge support and testing 

 screw, with photographic attachment; resurfacing grinding tools. 



Accessories and repairs of 60-inch reflector mounting and dome, as men- 

 tioned above. 



Patterns for large ruling machine and for mounting of lo-inch portrait 

 lens telescope. 



PASADENA OFFICE BUILDING. 



From the beginning of the Observatory's work, our appropriations have 

 been used mainly for instruments and working equipment, while the mem- 

 bers of the staflf have contented themselves with small and simply furnished 

 offices. As these were insufficient in number, and occupied space in the 

 shop buildings greatly needed for other purposes, a grant was requested last 

 year for an adequate office building, of fireproof construction. Erection was 

 begun in December, on a lot having a frontage of 100 feet, adjoining the 

 instrument-shop building on the east, after plans by Myron Hunt, architect. 

 The building, which is of concrete and brick, with roof of Spanish tile, is 

 now nearly ready for use. The first and second floors are devoted almost 

 exclusively to offices, 30 rooms being provided for the members of the ob- 

 serving staff and the computing division. The library, 24.5 by 36.5 feet, is 

 on the main floor. Underneath is a small stack room and a general labo- 

 ratory. The basement also contains a plate-storage room 43 by 21.5 feet, 



