eichelberger: distances op heavenly bodies 



173 



average probable error of a parallax was the same in each of 

 these two pieces of work, about 0".03. The progress of the work 

 during the last two or three generations is given in Table III 

 which contains also a brief statement of the discoveries made 

 during the preceding century due chiefly to efforts to measure 

 stellar parallaxes. 



TABLE III 



Approximate Number of Known Stellar Parallaxes 



A generation ago photography entered the field of stellar 

 parallax work, and has outdistanced all the previously employed 

 methods for efficiency. In 1911, two publications appeared 

 giving the results of photographic stellar parallax work, one 

 by Russell, giving the parallaxes of forty stars from photo- 

 graphs taken by Hinks and himself at Cambridge, England, the 

 other by Schlesinger, giving the parallaxes of twenty-five stars 

 from photographs taken mostly by himself at the Yerkes Obser- 

 vatory, Williams Bay, Wisconsin. In speaking of these two 

 series of observations, Sir David Gill said: 



On the whole, the Cambridge results, when a sufficient number of 

 plates have been taken, and when the comparison stars are symmetri- 

 cally arranged, give results of an accuracy which, but for the wonder- 

 ful precision of the Yerkes observations, would have been regarded as 

 of the highest class. 



Schlesinger has shown that with a telescope of the size and char- 

 acter of the Yerkes instrument "the number of stellar parallaxes 

 that can be determined per annum, with an average probable 

 error of 0''013, will in the long run be about equal to the num- 

 ber of clear nights available for the work." 



