MOUNT WILSON SOLAR OBSERVATORY. 1 69 



single stars. We hope to be able within a short time to secure lenses which 

 will give a much more uniform field, and to take up again the experiments 

 with the multiple-slit device. 



Since September 1909 Mr. Adams and Mr. Babcock have obtained about 

 460 photographs of spectra of 128 stars with this spectrograph. The great 

 majority of the negatives are of stars between the fifth and eighth magni- 

 tudes on the visual scale, and they have been used both for the purpose of 

 classification of spectral type and for the determination of radial velocity. 

 The stars observed are as follows: Kapteyn selected areas, 15; stars with 

 known parallaxes, 98; Scorpius group of Orion type, 10; miscellaneous 

 stars, 5. Almost all of the stars with known parallaxes have a type of spec- 

 trum resembling that of the sun. 



Three spectra of Halley's comet in the more refrangible part of the spec- 

 trum were obtained before its transit in May. 



The exposure-times with this spectrograph under moderately good condi- 

 tions are about as follows for stars of the solar type of spectrum : for stars 

 of visual magnitude 6.0, 12 minutes ; of visual magnitude 7.0, 30 minutes ; of 

 visual magnitude 8.0, 70 minutes. These results refer to the region of the 

 spectrum between A 4000 and A 4400. Spectra of Groombridge 1830, visual 

 magnitude 6.46, have been obtained in 15 minutes under good conditions. 



The measurement and reduction of a large number of these photographs 

 has been completed by Mr. Adams, and Miss Lasby and Miss Waterman 

 of the Computing Division. Miss Ware has also measured some of the nega- 

 tives. Where possible series of measures by two observers have been made 

 in order to reduce the accidental and personal errors inherent in this class of 

 work. Miss Waterman has carried on the greater part of the classification 

 of the spectra. 



Although, of course, the determinations of radial velocity with this instru- 

 ment are much inferior in accuracy to those made with powerful three- 

 prism spectrographs, the results obtained are satisfactory. The probable 

 error of a single line on a photograph of the spectrum of a star of the solar 

 type is about 3 km., or about 1 km. for the mean of the plate when 10 lines 

 are measured. The instrument, accordingly, provides the means of obtaining 

 with a fair degree of precision the radial velocities of stars to about the ninth 

 magniture on the visual scale. 



Photographic Investigations of the Spectra of Globular Star-Clusters and 



Spiral Nebula. 



In order to provide for work on very faint objects we have adapted the 

 low-dispersion spectrograph for use with a 30 light flint-glass prism and a 

 special lens, kindly loaned to us by the Yerkes Observatory, of 4 inches 

 aperture and 7.2 inches focal length. Mr. Fath has employed this instru- 

 ment during the past year and has obtained photographs of the spectra of 

 the following objects: 



Globular star-clusters : N. G. C. 5024, 5272, 6205, 6229, 6341, 6656, 6779, 6934, 7078. 

 Spiral nebulae: N. G. C. 650-1, 4725, 4736, 7331. 



