500 BURNS— RELATIVE AND ABSOLUTE PARALLAXES. 



There are several observatories whose parallax determinations 

 depend on comparison stars of about the same magnitude as those 

 used at Allegheny. The mean value of the parallax found by these 

 observatories was used in the following discussion. The excellent 

 work of the Mt. Wilson Observatory was not used because the com- 

 parison stars in that series are much fainter. The stars for which 

 the radial velocity, proper motion and parallax have all been observed 

 were grouped according to spectral class. The mean radial velocity 

 and proper motion was found for each class. This was not done by 

 plotting curves as in the case of the stars discussed in the first part 

 of the paper, but the simple arithmetic mean was used. Since the 

 data are fairly well represented by probability curves, formula (a) 

 may be used to derive the mean parallax. Table 2 gives the mean 

 observed (relative) parallax by classes in the second column. The 

 third column contains the mean values computed by formulae (a), 

 and column four shows the differences O — C. Column five shows 

 the same differences on the basis of a correction of o".oio to be ap- 

 plied to the observed relative parallaxes in order to reduce to absolute. 

 The results for Class F, indicating a larger correction are not entirely 

 above suspicion and were not used in obtaining the mean correction. 



TABLE 2. 

 Observed and Computed Mean Parallaxes. 



To many, a correction to reduce from relative to absolute paral- 

 laxes of the size of ten thousandths of a second will seem too 

 large. But we cannot avoid the conclusion that a correction of 

 this order must be applied to stars of Class B. Moreover, if the 

 size of the correction were influenced by instrumental causes it 

 would seem that Class M should indicate a correction considerably 



