MOUNT WILSON OBSERVATORY. 293 



man's time during about ten days per month is required for illustra- 

 tions of Observatory publications. The work has been done by 

 Messrs. Pease, Nichols, and Kinney. 



Optical Shop. 



The work of the optical shop has included the grinding and figuring 

 of the following mirrors: two 7.25 by 9 inch, plane; three 12-inch 

 diameter, plane; two 8-inch diameter, plane; one 4.5-inch diameter, 

 plane; one 14-inch concave mirror of 15 feet radius; also one 2-inch 

 quartz concave lens; optical parts for eight binocular eyepieces; quartz 

 disk for double-star interferometer; speculum-metal plates for gratings; 

 many small prisms and lenses. The following machines were designed 

 and partly completed: 6 spindle machines for lens work, small edge 

 grinder, optical testing apparatus. The work was done by Messrs. 

 Kinney and Dalton. 



Glass-Blowixg Shop. 



A small shop for making blown glass and quartz apparatus for spec- 

 troscopic and other purposes was equipped in July. Mr. D.J. Pompeo, 

 formerly with the Cooper-Hewitt Company, will devote part of his 

 time to this work. He is now making quartz tubes for use in the study 

 of the combined effect of magnetic and electric fields on radiation. 



Instrument Shop. 



The chief work of the instrument shop (Mr. Ayres, foreman) includes : 

 stereocomparator, large and small solenoid magnets, velocity of light 

 apparatus, 20-foot interferometer changes, extra focal interferometer, 

 vacuum chamber for large electric furnace, photographic recording 

 apparatus, glass-blowing equipment, spectrograph and concave-mirror 

 mounting of Snow telescope, 8 by 10-inch plate-holder for 60-inch 

 telescope, attachments for 10-inch refractor, binocular eyepieces; 

 and the following attaclmaents of the 100-inch telescope: Newtonian 

 mirror mounting, Cassegrain spectrograph VI, plate-holder, cage 

 clamp, declination slow-motion, traveling crane, observing platforms, 

 dome shutter, silvering equipment, instrument cabinets. Much 

 miscellaneous work was done on instrument repairs, construction for 

 the laboratory and optical shop, building repairs, and various minor 

 items. 



Ruling Machine. 



The accidental errors causing false spectra in the gratings ruled last 

 year were found to be due to the spacing mechanism. A new mechan- 

 ism designed and built by Mr. Jacomini this year has proved to work 

 perfectly, and the accidental errors appear to be completely removed. 

 The ruling carriage has been rebuilt and made very much lighter, 

 thus adding materially to the smoothness of operation of the machine. 



There remains nov/ a small and quite regular periodic error due to 

 slight residual eccentricities in the spacing wheel and the screw pivots. 



