264 CARNEGIE INSTITUTION OF WASHINGTON. 



telescope; large solenoid magnet and its auxiliary cooling-tank and 

 coils; a small star-diameter machine; and an 8 by 10 differential 

 measuring-machine for stellar parallaxes and proper motions. Mis- 

 cellaneous work has included designs for additions to the observing 

 platform at the Newtonian focus of the Hooker telescope, adapters for 

 plate-holders and spectrograph, cloth-covered extension of the 100- 

 inch tube, binocular eyepieces for measuring-machines, mirror supports 

 for velocity-of-light apparatus, many small attachments to existing 

 instruments, and many illustrations and charts. 



OPTICAL SHOP. 



A 36-inch concave mirror of 900-foot focal length and a 22-inch 

 plane were figured for the velocity-of-light apparatus. Other work of 

 the year included two 9-inch and six smaller plane mirrors, six quartz 

 interferometer plates, two 2.5-inch plane parallel plates, a 10-inch lens 

 for the finder of the Hooker telescope, and 15 grating-plates from 4 

 to 10 inches square. The opticians also assisted in the construction 

 of a 2-foot polishing-machine, an edge-grinder, a testing arrangement 

 for 4-inch plates, and a revision of the 12-inch testing-stand so as to 

 render it universal. 



The mirrors of the telescopes on Mount Wilson have been burnished 

 at regular intervals. 



INSTRUMENT SHOP. 



About half the time of the instrument shop has been devoted to the 

 Hooker telescope and its accessories, including the construction of the 

 20-foot stellar interferometer, smaller interferometer, observing plat- 

 form for work at the 134-foot Cassegrain focus, 8 by 10 plate-carrier 

 for the Newtonian focus, 4 by 5 and 5 by 7 plate-holders, and much 

 work on the Cassegrain spectrograph, temperature-control apparatus 

 for the 100-inch mirror, coude mechanism, tube extension, mountings 

 of Cassegrain and Newtonian mirrors, upper observing-platform, 

 ladders, instrument cabinets, besides many miscellaneous items. 

 Other work included the construction of three 4 by 10-inch machines 

 for measuring spectra, improvements in the 10-inch photographic 

 refractor, special arcs for the laboratory, improvements of the coelostat 

 and spectrograph of the Snow telescope, grinding and polishing 

 machines for the optical shop, installation of 500-k. w. motor gener- 

 ator, large solenoid magnet and cooling-plant, velocity-of-light appa- 

 ratus, earth-tide apparatus, and miscellaneous equipment and repairs. 



RULING MACHINE. 



Several gratings have been ruled during the year, some of which 

 give excellent resolution and unimportant ghosts, but show con- 

 siderable diffuse Ught due to a series of short spectra. These probably 

 result from some obscure source of periodic error, which is now being 

 sought by Mr. Jacomini. 



