216 CARNEGIE INSTITUTION OF WASHINGTON. 



and the variations of the so-called earth light with zenith distance, 

 azimuth, and time. The preliminary reductions are promising, but 

 further observations of the earth light are needed before the bright- 

 ness of the starhght in the Milky Way can be determined definitely. 



Stellar Spectroscopy. 



INSTRUMENTS. 



Most of the stellar spectroscopic observations of the past year 

 have been made with the Cassegrain spectrograph at the 80-foot focus 

 of the 60-inch reflector. For stars brighter than magnitude 6.5, a 

 64° prism with a camera of 102 cm. focal length has been used, 

 and for fainter stars the same prism with a 45.7 cm. camera. Re- 

 cently a prism of 39° angle was completed in the optical shop, and 

 preliminary tests with this prism and a triple camera objective of 

 45.7 cm. focal length indicate that spectra of stars of magnitude 

 9.5 may be obtained with an exposure time of about 2.5 hours. The 

 scale of the photographs is sufficient to enable excellent classification 

 as well as to provide approximate radial velocities. 



During the year two new spectrographs designed for special obser- 

 vational work have been completed. One of these is a concave- 

 grating spectrograph having a grating of 1 m. radius. It is designed 

 so that it may be used in either the first or second order and with or 

 without a collimating lens to remove astigmatism. It is planned to 

 use this instrument in the detailed study of certain special types of 

 stellar spectra. 



The second instrument is a very small spectrograph planned for 

 work on extremely faint stars and nebulae, at the primary focus of 

 the telescope. It consists of a 60° ultra-violet glass prism used in 

 conjunction with camera and collimating lenses of 27 mm. aperture 

 and 136 mm. focal length. The spectrograph fits directly into the 

 plate-holder opening and the guiding is carried on in the same way 

 as for direct photography, by moving the spectrograph on the double 

 slides by means of the guiding screws. This instrument has been 

 employed by Mr. Pease in the photography of the spectra of several 

 faint stars and nebulse. 



RADIAL VELOCITIES. 



The program of work during the year has consisted mainly of a 

 continuation of the determination of the radial velocities of the two 

 lists of stars already under observation last year. The first is made 

 up of stars having spectra of types A and B, a knowledge of whose 

 velocities is of especial importance for the investigations by Professor 

 Kapteyn upon star-groups and star-streaming. The second list 

 consists of stars of large proper motion for which the parallaxes have 

 been determined. A knowledge of the radial velocities of these stars 



