MOUNT WILSON SOLAR OBSERVATORY. I7I 



axis of the large mirror. The loss of light at the diagonal plane mirror will 

 thus be avoided, an especially important consideration in studies upon ultra- 

 violet spectra. This frame is planned to carry a small spectrograph for work 

 upon stars below the seventh magnitude, a very short-focus, low-dispersion 

 spectrograph for investigations of the spectra of nebulae and star-clusters, 

 and a quartz spectrograph of 2 inches aperture for ultra-violet spectra. The 

 frame is designed to admit of the use of a plate-holder for direct photog- 

 raphy as well. 



Direct Photography. 



The photography of the Kapteyn Selected Areas with and without absorp- 

 tion screens has been continued during the year by Mr. Fath, and is now 

 practically completed. In addition a few plates have been taken with colored 

 screens for the purpose of studying the possible effect of the absorption of 

 light in space. Since observing conditions during the winter months as a 

 rule are comparatively inferior, the direct photography of nebulae and star- 

 clusters was discontinued between the months of December and June, the 

 additional time available being used for spectrographic work. In July a 

 series of photographs of certain selected nebulae and clusters was commenced 

 by Mr. Pease, with a view to obtaining the material for a study of the stellar 

 distribution within these objects. The photographs should prove of great 

 value in the interpretation of the spectrographic observations. 



Measures made under the direction of Professor Kapteyn of a few photo- 

 graphs taken at the primary focus of the reflector for the determination of 

 stellar parallaxes indicate that, while the order of precision is high, the linear 

 scale is insufficient for this very exacting type of work. Accordingly there 

 is now under construction in the instrument-shop a large metal plate-holder 

 and support, to be used at the 100-foot equivalent focus, and with the com- 

 pletion of this instrument the work will be taken up again. 



Photographic Photometry. 



The investigation of methods of determining photographic magnitudes 

 with the 60-inch reflector has been continued throughout the year by Mr. 

 Seares, and about 280 photometric plates have been secured by him. Of 

 this number, 21 are of the North Polar Sequence and of the four secondary 

 sequences at +15° declination, and i, 7, 13, and 19 hours of right ascension. 

 These were taken at the request of Professor Pickering and have been 

 forwarded to the Harvard College Observatory for measurement. 



The remaining plates relate mainly to an investigation of the diffraction 

 eft'ect that enters when the photographic magnitude scale is established 

 through a comparison of exposures made with the full aperture and with 

 diaphragms of various shapes and sizes, and to a study of the correction 

 required by the observed magnitudes, which depends upon the distances of 

 the individual stars from the axis of the instrument. 



The diffraction effect has been studied from two different standpoints. 

 The first depends upon an actual comparison of the magnitude scales estab- 



