402 ANNALS NEW YORK ACADEMY OF SCIENCES 



its effect on the measured diameter. The insertion of a prism in the 

 Repsold hehometer reduced the apparent diameter by 0''A. The eariier 

 observations of Schur and Ambronn, as given by Auwers, were 



Schur 1919".52 



Ambronn 1919^.26 



But at the time of these observations other observers made the diameter 

 vary between the limits 



1920".29 and 1918''38. 



and Auswers found systematic errors depending upon the instrument used. 



Part II. Photographic Measures. 



The following investigation of the figure of the sun was suggested by 

 the number of solar photographs taken by Lewis M. Rutherfurd in his 

 private observatory, and by him presented to the Observatory of Columbia 

 University. In a series of investigations of the Rutherfurd star plates, 

 Jacoby has shown that the plates have suffered no deterioration, and that 

 they are capable of giving results comparable in accuracy with the best 

 heliometer determinations. It was hoped, therefore, that the Rutherfurd 

 photographs of the sun would serve to determine with great precision the 

 shape of that body, and in 1905 the writer undertook the measurement and 

 reduction of all the Rutherfurd plates. 



After the investigation was well under way and some preliminary results 

 were obtained, it was decided to enlarge the scope of the investigation so as 

 to include all available photographs. Unfortunately it was soon found that 

 the number of suitable photographs is very limited. While several observa- 

 tories have long series of solar photographs, yet in nearly every case these 

 were taken wuth a horizontal telescope and without the necessary precau- 

 tions to secure an edge sufficiently sharp for measurement. The mirror 

 of horizontal instruments introduces errors and makes the solar image 

 unsymmetrical. Only such plates as have been made with an equatorially 

 mounted objective of relatively long focus can be used, and there are no 

 long series of such plates. 



Direct photographs of the sun's disk serve to determine the shape of 

 that body, but cannot well be used to determine its absolute diameter. 

 Changes in temperature affecting the focal length of the objective cause 

 variations in the scale of the photographs. In stellar photography the scale 

 of each plate is determined from measures of known stars whose images 

 are found on that plate; in solar photography the plates do not contain 



