174 



PROCEEDINGS OF THE AMERICAN ACADEMY 



If, therefore, the probahle error can be taken as a measure of the 

 accuracy of the observations, there ought to be no difficulty in obtain- 

 ing, from a moderate number of observations, the right ascension 

 within .02 9 - and the declination within 0".2. Yet it is doubtful, after 

 continuous observations in all parts of the world for more than a cen- 

 tury, if there is a single star in the heavens whose absolute co- 

 ordinates are known within these limits. In 1866 the illustrious 

 Argelander proposed a list of stars for simultaneous observation by 

 different observers, for the purpose of investigating the systematic 

 differences which he found to exist in all modern catalogues. This 

 scheme was carried out only to a limited extent. But in 1878 the 

 fortunate requirements of a special problem secured data which will 

 go far towards the establishment of the existence of these errors, even 

 with the present methods of refinement in observation, if indeed 

 they do not for the present reveal their cause. 



During that year Mr. David Gill, recently appointed Director of the 

 Cape of Good Hope Ohservatory, solicited the co-operation of astron- 

 omers in determining the co-ordinates of 28 stars, which he used in 

 his heliometer observations of the planet Mars for obtaining the solar 

 parallax. The observatories named below made the observations 

 required, which were forwarded to Mr. Gill upon the completion of 

 the reductions. The results are published in Vol. XXXIX., page 99, 

 of the Monthly Notices of the Royal Astronomical Society. 



In the following table are given the differences between the least 

 and the greatest results for each star, both in right ascension and in 

 declination. 



Even after the observations were reduced to a homogeneous system, 

 Mr. Gill finds the following outstanding errors : — 



