256 CARNEGIE INSTITUTION OF WASHINGTON. 



motions and radial velocities of the stars of both the First and Second 

 Streams. This more extensive investigation has brought to hght con- 

 tradictions which have not yet been removed, so that the acceleration 

 of the First Stream can not yet be considered as a well established fact. 



In the meantime, the work thus far done proves the continuity of 

 the series of the B and A and the second-tjnpe stars. Hitherto, the 

 B stars have seemed to occupy a place by themselves, inasmuch as they 

 apparently formed but a single stream, coinciding neither in direction 

 nor in velocity with either of the two main star-streams. In the 

 Reports for 1911 and 1912 it was stated that some B stars at least must 

 be regarded as members of the Second Stream. It is now found that 

 the remainder — the overwhelming majority — belong to the First 

 Stream. The continuity of the results for the B and A and the second- 

 type stars appears as soon as the stars are arranged (roughly) in order 

 of distance. 



2. The second undertaking has been the completion of the investi- 

 gation of the individual parallaxes of the B stars between galactic 

 longitudes 150° and 216° and galactic latitudes -30° and +30° (see 

 Report for 1914). This study has now been finished and can be in the 

 printer's hands in a few weeks. It has resulted in a fairly good deter- 

 mination of the distances of 166 of the 168 Boss stars of spectrum B in 

 this region. The stars in a part of the region — that round the Great 

 Nebula (and probably belonging to the same system) — form a local 

 group for which the parallax could not be detemiined in the same way 

 as for the others. Nevertheless a fairly satisfactory result was found 

 by a somewhat indirect method. 



The paper will contain separate luminosity curves for the stars of 

 spectrum BO, B3, B5, BO to B2, BO to B3, Bl to B2, BO to Bo, B5 to B9, 

 B8 to B9, and AO to A9. 



The measurement of the Mount Wilson photographs of the Selected 

 Areas (exposure I'') has been completed in the Groningen Laboratory. 

 The reduction of the positions is practically finished. The reduction 

 of the magnitudes will be taken in hand as soon as the standard 

 magnitudes, now being determined at Mount Wilson, are complete. 



PHYSICAL LABORATORY. 

 INSTRUMENTS. 



An auxiliary concave-grating spectrograph constructed for use with 

 the interferometer has proved highly efficient. The grating is of 21 feet 

 radius of curvature and has 87,500 lines, spaced 15,000 to the inch. 

 It is furnished with a collimated beam of Hght from an 8-inch silver- 

 on-glass mirror of 21 feet radius of curvature, thus making a compact 

 instrument free from astigmatism. The mounting is supported upon 

 three cement piers set upon the laboratory floor, the principal parts 

 being connected by heavy channel irons. The pier carrying the slit and 



