MOUNT WILSON SOLAR OBSERVATORY. 285 



rough-grinding four of the smaller mirrors (two plane, two convex) 

 for the 100-inch telescope. 



Considerable time has been spent in experimental work in grinding, 

 polishing, and figuring speculum metal, in the endeavor to apply to 

 this work improved methods of grinding and polishing which have 

 proved highly successful and economical in working glass. 



WORK OF THE INSTRUMENT SHOP. 



After the completion of the large Koch registering microphotometer 

 last September the chief work of the instrument shop (Mr. Jacomini, 

 chief instrument maker; Mr. Ayers, foreman) included the continua- 

 tion of the ruhng-machine, the 100-inch telescope mounting and dome- 

 drive mechanism, the 10-inch portrait-lens telescope, and three small 

 domes; the machining of the rails for the dome of the 100-inch tele- 

 scope; the remodeling of the 60-foot tower telescope and 30-foot 

 spectrograph; and the construction of a 13-foot spectroheUograph, a 

 mounting for the 5-foot plane mirror used in testing the 100-inch mir- 

 ror and various new parts for the 100-inch grinding machine, a 20,000- 

 volt power plant and other apparatus for the Stark effect, a water tank, 

 air-pressure tank and apparatus for vortex experiments, a 4 by 5 plate- 

 measuring machine, and much miscellaneous work, including minor 

 apparatus for the laboratory, additions to the Cassegrain and concave- 

 grating stellar spectrographs, new shop equipment, the installation of 

 safety devices in the shop, instrument repairs, and repairs of power 

 plant and auto-trucks. Some work was also done for the Smithsonian 

 Astrophysical Observatory on Mount Wilson. 



Mr. Jacomini has carried the ruling-machine to a point where the 

 actual ruUng of gratings may be begun. The experiments of the year 

 have led to some minor modifications in the driving mechanism and 

 the lightening of the grating carriage. Through the courtesy of 

 Dr. Kunz, for which we are greatly indebted, Mr. Jacomini was given 

 an opportunity to study the methods of diamond cutting practiced 

 in the shops of Tiffany and Company, where he gained much valuable 

 information. Recent experiments in cross-ruling are so satisfactory 

 that the ruhng of gratings will now be undertaken. Speculum metal 

 plates are being figured for this purpose in the optical shop, and experi- 

 ments on the use of other alloys have also been made. 



The chief addition to shop equipment made during the year is a 

 Rivett precision screw-cutting lathe for small work. 



ONE-HUNDRED-INCH TELESCOPE. 



Much delay has been experienced at Fore River in the work on the 

 100-inch telescope mounting, and the first shipment of parts of the 

 instrument, though promised last spring, has just been made. The 

 problem now is to get these heavy pieces (some of them weighing 

 over 9 tons each) to the summit of Mount Wilson before the rainy 



