MOUNT WILSON OBSERVATORY. 259 



concave speculum metal mirrors, ranging in diameter from 4 to 10 

 inches, for grating plates and other purposes; two 6-inch test planes; 

 two right-angle prisms. Two quartz prisms and one glass prism have 

 also been refigured, and much miscellaneous optical work has been 

 done. 



THE INSTRUMENT SHOP. 



Prior to the armistice, the instrument shop (Mr. Ayers, foreman; 

 Mr. Jacomini, chief instrument maker) was almost exclusively en- 

 gaged with Government work, as detailed in a separate report on the 

 war services of the Observatory. In fact, including overtime, 57 

 per cent of the shop-work during the year was for the Bureau of 

 Standards, the Signal Corps, the Ordnance Department of the Army, 

 and the Navy Department. 



Since the armisitice, excepting for the completion of the optical 

 work carried through until April at the request of the Ordnance De- 

 partment, the instrument shop has been engaged on the regular work of 

 the Observatory. That chargeable to the Hooker telescope includes 

 work on the Cassegrain spectrograph, double-slide plate-holder, tube- 

 balancing system, coude mechanism, cage-clamps, mirror temperature- 

 control, muTor-silvering equipment, Newtonian and Cassegrain cages 

 and mirror mountings, driving-clock, sidereal indicator, burnishing 

 apparatus, observing platform, dome ladders and stairs, and dark-room 

 equipment. The other chief items of construction relate to the 

 coelostat and spectrograph of the Snow telescope, a focal plane spec- 

 trograph, stellar comparator, laboratory grating spectrograph, Smith- 

 sonian Observatory equipment, and instrument and building repairs. 



RULING MACHINE. 



The work on the ruling machine during the early part of the present 

 year revealed the fact that the spacing wheel (a steel forging) had 

 warped somewhat since it was first made. The tedious work of 

 correcting this occupied Mr. Jacomini some three months, but was 

 perfectly successful. Cross-ruling tests were then made which con- 

 clusively showed the machine free from periodic errors. There were, 

 however, accidental errors of rather large amount. After a lengthy 

 process of elimination their cause was found in the casting supporting 

 one pair of the ruling-carriage ways. This casting has now been 

 strengthened and supported in such a way that it is hoped no further 

 difficulty will occur. These changes, however, necessitated a regrind- 

 ing of the ways in order to make them again straight and parallel. 

 This work is now nearly completed. 



ONE-HUNDRED-INCH HOOKER TELESCOPE. 



The 100-inch Hooker telescope, though greatly delayed by the war, 

 is now in regular use. It will therefore be of interest to give some 



