MOUNT WILSON SOLAR OBSERVATORY. 167 



(3) The nebulosity is in general much brighter toward the center of the 

 nebula, becoming gradually fainter toward the extremities of the branches. 

 In the case of the nebulous stars, however, this tendency is less marked and 

 they are often brighter and more numerous in the intermediate region be- 

 tween the center and the extremities of the branches. 



(4) The nebulous stars are frequently visible in the extremities of the 

 branches beyond the point at which the nebulosity ceases to be visible. These 

 stars frequently occur in groups. In Messier 33, for example, which contains 

 over 2,400 nebulous stars, there are at least 20 such groups, each containing 

 from 10 to 60 stars. 



Photographic Photometry. 



Systematic investigations in photographic photometry have been under- 

 taken by Mr. Scares. Fifty plates of the north pole and one other specially 

 selected region have been obtained with the 60-inch reflector, mainly with 

 wire-gauze screens and diaphragms of various forms and sizes. Ten addi- 

 tional plates of the north pole have been taken with the 60-inch reflector by 

 Mr. Scares and Mr. Fath at the request of Professor Pickering for use in 

 the determination of the absolute magnitudes of the fainter stars of the Har- 

 vard Polar Sequence. 



At present the relation between diffraction pattern and the corresponding 

 photographic image is being studied with a view to determining the relia- 

 bility of magnitudes derived through the use of diaphragms. The results 

 thus far obtained indicate that the systematic differences between magnitude 

 scales based on exposures with the full aperture combined with those with 

 diaphragms of ^2, 14, and 6 inches, respectively, are at least no greater than 

 the uncertainty of the determination of the scales themselves from a single 

 plate. Whether there exist small systematic differences at present masked 

 by the uncertainties mentioned remains yet to be seen. As soon as it is clear 

 that a reliable absolute photographic magnitude scale extending to the fainter 

 stars has been established it is proposed to undertake a determination of the 

 brightness of the stars of the Pritchard-Kapteyn regions. 



Steli^ar Spectroscopy. 



Two stellar spectrographs have been in use throughout the past year, the 

 high-dispersion 18-foot instrument described in the last annual report and 

 a small low-dispersion spectrograph used at the primary focus with the 

 Newtonian combination of mirrors. The large three-prism spectrograph 

 which has been under construction by William Gaertner & Co. is now in 

 Pasadena, and will soon be ready for use. It will be employed with a Casse- 

 grain combination of 80 feet equivalent focal-length. 



The observations with the 18- foot spectrograph have been carried on by 

 Mr. Hale, Mr. Adams, and Mr. Babcock, and spectra of the following stars 

 have been obtained : a Ononis, ^ Ononis, Sirius, Procyon, Arctiirns, and 

 Antares. The photographs in the cases of Sirius and ArcHirus cover the 



