MOUNT WILSON OBSERVATORY. 247 



possible hitherto, on account of the inadequate character of the exist- 

 ing parallax material upon which to base the reductions. The absolute 

 magnitudes and parallaxes of about 1,800 stars have now been derived 

 upon the revised system and will soon be ready for publication. 



As a by-product of the investigation of mean parallaxes and mean 

 proper motions, reference may be made to the fact that the mean paral- 

 lax derived spectroscopically for groups of stars of definite apparent 

 magnitude does not become zero when the proper motion becomes 

 vanishingly small, but tends toward a finite limit. A study of this 

 deviation from Kapteyn's formula shows that this peculiarity is a 

 necessary consequence of the grouping according to proper motion, 

 and of the fact that there is a definite upper limit to the luminosity of 

 a star. 



Stars with Spectra Characteristic of the Cepheid Variables. 



In the course of the classification of stellar spectra, 18 stars 

 have been found with spectra very similar to those of the Cepheid 

 variables. They are characterized by very strong hydrogen lines and 

 enhanced lines which are several times as intense as in normal stars 

 of the type to which these stars belong. Many of the spectra are 

 essentially identical with that of typical Cepheids such as 5 Cephei and 

 ^ Geminorum, and it seems clear that the relationship to the Cepheids 

 must be intimate, although no variation of light is known with certainty 

 in any case. 



An examination of the list shows that these stars resemble the 

 Cepheids in the following respects as well: (1) very small proper 

 motion; (2) low galactic latitude; (3) small radial velocity; (4) very 

 high intrinsic luminosity. At least 6 of the stars show a small variation 

 in radial velocity. 



It seems probable that a further study of the spectra of such stars 

 may aid in the solution of the problem of Cepheid variation. 



The Spectrum of Nova Aquil^e No. 3. 



Photographs of the spectrum of Nova Aquilse have been made 

 regularly throughout the year. In addition to the usual changes 

 characteristic of Novae, the spectrum has shown remarkable peculiar- 

 ities, especially in the form and structure of the emission bands. 

 Some of the more important results of detailed study of the photo- 

 graphs are briefly: 



(1) A large number of absorption lines identified in the spectrum of 

 Nova Aquilse have also been identified in that of Nova Geminorum 

 No. 2. Their displacements are almost exactly one-half as great as in 

 Nova Aquilse. 



(2) The displacements of the two components of the hydrogen and 

 helium lines in Nova Aquilse are in the ratio of 3 to 2. A comparison 

 with the three other principal Novse of recent years, Nova Aurigse of 



