228 CARNEGIE INSTITUTION OF WASHINGTON. 



Photometry. This discussion and its results have been published in Con- 

 tribution No. 235. 



Mr. Seares has given much time to the preparation of the report of the 

 Commission on Photometry for the meeting of the International Astronomical 

 Union at Rome. Eight separate determinations of the photographic scale, 

 made at six different observatories, were combined to form a series of standard 

 magnitudes which has been adopted by the Union. The Harvard results 

 for stars fainter than the sixteenth magnitude were omitted at the request of 

 Miss Leavitt. The Mount Wilson data, which extend to the twentieth photo- 

 graphic magnitude, are accordingly the only ones available for very faint 

 stars, and until this portion of the scale receives independent confirmation 

 it must be regarded as provisional. For stars brighter than the sixteenth 

 magnitude, however, the agreement of the different investigations is very 

 satisfactory. The average deviation of a final magnitude for any one obser- 

 vatory, between the limits 2.6 and 16.0, is ±0.024 mag. 



The Mount Wilson measures of photovisual brightness are the only ones 

 extending to the faint stars (17.4 photovisual). The scale stands in the proper 

 relation to the photographic scale, however, for the color-indices obtained by 

 comparing magnitudes show no appreciable systematic deviation from those 

 obtained by the method of exposure ratios. 



STELLAR SPECTROSCOPY. 



Nearly one-half the time of the two large reflectors has been devoted to 

 stellar spectroscopy, and numerous observers have shared in the work of this 

 department. Among the principal lines of work carried on during the year 

 are the following: 



General program of radial velocities, determinations of absolute magnitude, and 

 investigations of the spectra of certain classes of variable stars: Adams, Joy, 

 Stromberg, Hoge. 



Variables of type Me and Se and other stars with bright lines: Merrill, Humason. 



Spectroscopic binaries and R-type stars: Sanford. 



Nebulous stars: Hubble. 



Stars of Kapteyn's Selected Areas: Humason. 



Most of the spectrograms have been obtained with two slit-spectrographs 

 at the Cassegrain focus of the reflectors. These have remained without 

 change during the year, except for the recent addition to the spectrograph 

 of the 100-inch telescope of a camera lens by T. Cooke and Sons of 6.3 cm. 

 aperture and 12.8 cm. focal length. In the case of extremely faint objects 

 this wide-angle lens should prove of considerable value. 



The small spectrograph employed at the primary focus of the telescopes 

 has been adapted by Mr. Sanford for use with two prisms and a lens with 

 very short focus and excellent results have been obtained. Another spectro- 

 graph, for use without a slit, by means of which the spectra of a number of 

 stars may be obtained at one time for classification purposes, was completed 

 during the winter. With this instrument Mr. Humason, working at the prim- 

 ary focus of the 60-inch reflector, has succeeded in classifying spectra of stars 

 in Kapteyn's Selected Areas down to the twelfth magnitude on the photo- 

 graphic scale. 



Two other spectrographs, both for use at the Cassegrain focus of the 

 100-inch reflector, have recently been added to our equipment. One of these 



