MOUNT WILSON OBSERVATORY. 



277 



It is easily shown that the total mass of 



the various spectral types. 

 the system ix is given by 



logM =0.3-0.6 {M-M,) 



where M is the true absolute magnitude and M^ the value correspond- 

 ing to the hypothetical parallax ttc, calculated in the usual manner with 

 an assumed value /x=2. The results of Jackson and Fumer (Monthly 

 Notices, R. A. S., vol. 81, 2, 1920) provide values of M^ for more than 

 550 systems. The individual values of M are mostly unknown. But 

 the mean absolute magnitudes for a group of binary systems will be 

 the same, presumably, as that of a similarly selected group of non- 

 binaries. Abundant material for the latter is available in the cata- 

 logue of spectroscopic parallaxes and the lists of helium stars whose 

 luminosities were derived by Professor Kaptejoi. 



The results for the geometrical mean masses and the mean absolute 

 magnitudes to which they correspond are in the accompanying table. 

 The values for types earlier than A5 and for type M are affected by 

 considerable uncertainty. 



For the later spectral types the masses are those of the dwarfs. 

 About a hundred of the binaries occur in the list of spectroscopic 

 parallaxes, and these fully confirm the rate of variation of mass with 

 type for the interval FO to K5. 



STELLAR INTERFEROMETER. 

 Interferometer Measures of St.\r Diameters. 



In the last annual report a 20-foot interferometer, designed and built 

 for use with the 100-inch telescope, was briefly described. The pre- 

 liminary tests and adjustments of this instrument were completed 

 in September by Messrs. IVIichelson and Pease. After Professor 

 Michelson's return to the University of Chicago, Mr. Pease continued 

 to make systematic observations, and on December 13 he succeeded 

 in measuring the angular diameter of a Orionis. The fringes were 

 found to vanish at a mirror separation of 10 feet, and, subsequently, 

 corresponding determinations for a Bootis and a Scorpii were found 

 at 21 and 12 feet, respectively. Assuming the mean wave-lengths for 



