236 CARNEGIE INSTITUTION OF WASHINGTON. 



tant refinements in the design and construction of the long nut, 

 which was counter-weighted in order to relieve the pressure due to 

 its considerable weight. The tests made from time to time during 

 the process of grinding were extremely satisfactory, and a steady 

 decrease in the errors was observed. The radius of the axis of the 

 screw is over 200 miles, corresponding to a bend in the screw of less 

 than 0.000001 inch. The bearings are now parallel to the axis 

 within 0.00002 of an inch, but this quantity will be reduced later to 

 one-quarter of its present amount. A preliminary interferometer 

 test, made after the completion of the grinding and polishing, showed 

 no periodic errors greater than 0.000001 inch, and no appreciable 

 error of run has yet been detected. The maximum error in the teeth 

 of the worm gear does not exceed 0.001 inch, which is too small 

 to produce appreciable "ghosts." The definitive adjustments and 

 tests of the machine will involve many months of cross-ruling. At 

 present Dr. Anderson and Mr. Jacomini are devoting their atten- 

 tion to a study of various materials for end-thrust bearings, which 

 are being tested experimentally under conditions exactly similar to 

 those encountered in ruling. Hardened steel, agate, and ruby sur- 

 faces are being employed with the most promising results. 



Meanwhile the less exacting work on the ruling machine has pro- 

 ceeded satisfactorily in the larger instrument-shop. The pattern 

 work was begun in September, but for some months after the castings 

 were received the machine work could not be pushed rapidly on 

 account of the necessity of seasoning them by alternate exposure to 

 heat and cold. Moreover, the large planer, not then installed, was 

 needed for planing and grinding the bearings of the bed and cross- 

 carriage. Recently the work has gone forward very rapidly, and the 

 machine is now being assembled. 



In addition to the construction of the ruling-machine the instru- 

 ment shop has done the following work during the year: 



15-foot concave-grating spectrograph, mercury contacts for tuning-fork, prism mounting, 

 and generator mounting for physical laboratory 



30-foot and GO-foot lens mounting, liquid prism, additions to 75-foot spectrograph, and 

 other minor work and repairs for the 150-foot tower telescope. 



Completion of concave-grating spectrograph, small focal-plane spectrograph, and other 

 additions and repairs to the 60-inch reflector and its accessory instruments. 



Table-support, glass-testing mechanism, roller-bearing support, knife-edge, and other 

 additions to the 100-inch grinding-machine equipment. 



Focal-plane spectrograph, structural iron work on south pier, and other work in connec- 

 tion with the 100-inch reflector. 



Continuation of work on 10-inch photographic telescope. 



Miscellaneous accessories and repairs,work for mountain power plant, etc. 



In addition to the above, the employees of the instrument-shop 

 have devoted much time to the alteration and additions to the shop, 

 including the cleaning and installation of new machinery, the mov- 

 ing of some of the older machines, and the construction and erection of 

 cranes, tools, and attachments in preparation for the work on the 



