240 CARNEGIE INSTITUTION OF WASHINGTON. 



are chiefly associated with high luminosities. On the other hand, 

 assuming the relationship between luminosity and radial velocity 

 found for the stars of advanced spectral type to apply also to plan- 

 etary nebulae, the high radial velocities observed for these objects, 

 averaging about 29 km. per second, are in harmony with their low 

 intrinsic brightness. The mean diameter of the 6 planetary nebulae 

 for which parallaxes are now known is 4,000 astronomical units or 

 0.06 light year. 



For the F-type star of magnitude 12.34 and annual proper motion 

 3'/01, found by Mr. van Maanen in 1917, a relative parallax of +0''244 

 has been derived, corresponding to an absolute magnitude of +14.3 

 photovisual, or + 14.8 photographic. It appears, therefore, to be by 

 far the faintest F-type star known at present. If its surface bright- 

 ness be assumed to correspond with the average for F-type stars in 

 general, the observed parallax gives a diameter no greater than that of 

 the earth. 



Two fields have been examined by Mr. van Maanen for proper 

 motion : 



(1) A region in the Pleiades, for which a plate taken in 1913 at the 

 80-foot focus of the 60-inch telescope was compared with two exposures 

 made in 1918. 85 stars were measured, the faintest being of photo- 

 graphic magnitude 15.7. In order to secure the best results, the quad- 

 ratic terms in x and y were not neglected in the reductions. The 

 proper motions resulting from two pairs of plates may be estimated to 

 have a probable error in each coordinate of 0V018 divided by the num- 

 ber of years in the interval. Of the 85 stars measured, only 5 show 

 proper motions equal to that of the Pleiades ; the faintest of these is of 

 photographic magnitude 13.7. Adopting 0'/015 as the parallax of the 

 group, the absolute magnitude of this star is +9.6, as contrasted with 

 — 0.9 for the brightest star of the Pleiades. 



(2) A region in the Orion Nebula. Mr. van Maanen's measures 

 of two pairs of plates taken with the 40-inch Yerkes refractor were 

 improved by using the quadratic terms in x and y, a magnitude cor- 

 rection, and a correction from relative to absolute motion. The results 

 show that the refractor plates, as well as those made with the reflector, 

 require the use of quadratic terms. 



Assuming that the numerous variable stars in this region belong 

 to the Orion Nebula and move with it, the resulting proper motion 

 of the nebula is 0''006 in position angle 110°. The background stars 

 seem to show a decided preferential motion in the direction of Kap- 

 teyn's first stream. Much more material for different parts of the 

 sky will of course be necessary before any conclusions can be drawn 

 regarding the participation of stars as faint as the fourteenth and 

 fifteenth magnitudes in the stream-motion. 



