MOUNT WILSON SOLAR OBSERVATORY. 1 93 



followed by a double-image prism supported just in front of the photo- 

 graphic plate. In the laboratory this arrangement gave excellent results with 

 extremely weak fields. The brightest planetary nebula available when the 

 apparatus was ready in August was 2 6, and this was west of the meridian. 

 The chief nebular line, though faintly visible, gave no trace on the plate with 

 an exposure of 3 hours. The experiment will be resumed, with much longer 

 exposures, when suitable objects become available. 



INTEGRATED SPECTRUM OF THE MILKY WAY. 



As noted in the last report, an exposure of about 65 hours was made by 

 Mr. Fath with a small spectrograph on the bright portion of the Milky Way 

 in Sagittarius. The negative indicated a solar-type spectrum. It was feared 

 that this result might be due in part to a number of K-type stars in the 

 vicinity, but as a photometric investigation has shown this fear to be ground- 

 less, it seems probable that the greater portion of the light from this part 

 of the Milky Way comes from solar-type stars. 



Another exposure of about 68 hours was made on another bright portion 

 of the Milky Way in Scutum Sobieski. This plate shows exactly the same 

 type of spectrum as the preceding. A third exposure of 74 hours of a 

 region in Cygnus is essentially like the other two. 



The two negatives, indicating that the greater part of the light from those 

 regions of the Milky Way comes from G-type stars which are fainter than 

 those studied by Pickering, may explain the result noted in the preceding 

 section that the fainter stars of the Milky Way are redder than the brighter 

 ones. 



Photometric observations to determine the brightness of the sky, carried 

 on through most of the year by Mr. Fath, are being continued by Mr. Van 

 Rhyn. 



PROFESSOR KAPTEYN'S INVESTIGATIONS. 



The study of the motions of the helium and the A to A 3 stars has been 

 continued. The following are the main features of the investigation during 

 the year : 



( 1 ) Owing to the numerous determinations of radial velocity which have 

 become available this year, mainly through the labors of the Mount Wilson 

 and the Lick Observatories, a new and for the first time fairly reliable de- 

 termination of the velocities of the helium stars, Stream I, and the A 

 stars, Streams I and II, was carried through. The determination of this 

 last velocity depends almost entirely on the Mount Wilson observations, 

 which were made expressly for the purpose. The positions of the vertices 

 were also greatly improved. 



(2) As was stated in last year's report, the second stream should not be 

 completely absent from the helium stars. This year a few more stars were 

 found apparently belonging to the stream. The principal gain, however, 

 is due to Mr. Kohlschiitter's classification. As a result of his investiga- 



