POOR, THE FIGURE OF THE SUN 399 



a quantity which agrees closely with that (0'^.038) obtained by Auwers 

 from the transit of Venus observations. The mean errors of the individual 

 results are given by Ambronn as 



From Schur ±0''.021 



From Ambronn iC-OlS 



And these are nearly, if not quite, as large as the quantity sought. Ambronn 

 concludes, therefore, that the deviations are accidental and that the sun is 

 sensibly a sphere. 



In testing this result, Ambronn investigates the effect of the inclination 

 of the measured diameter on the result to determine whether there was any 

 tendency on the part of the observer to measure vertical diameters differ- 

 ently from horizontal. 



He could find no such effect; but he calls special attention to the obser- 

 vations made during the two years 1890 and 1891, which show the polar 

 diameter to be decidedly the greater, and points out the fact that these 

 results may be due to physiological causes, for during this interval no pre- 

 cautions were taken to obviate this difficulty. A prism was attached to 

 the heliometer in October, 1891, in such a manner that all the diameters of 

 the sun were measured in the same relative positions as regards the vertical; 

 and from that date on, the observations are perfectly homogeneous. 



Ambronn also investigates the possibility of errors in the constants of 

 refraction which were used in reducing the observations. In the winter 

 months the sun was at an average lower altitude at the time of observation 

 than in the summer months. Hence, if there were any systematic errors 

 in computing the differential refraction, such errors would be apparent 

 when the observations are grouped according to the months in which they 

 were made. When the observations are so grouped, no periodic variation 

 is shown; and Ambronn concludes, therefore, that the constants and the 

 methods used in computing the differential refraction are sensibly correct. 



2. Variation of the Spin's Diameter. — Each series of observations is 

 treated separately. Ambronn first finds the mean value of the sun's diameter 

 from all the observations of each series; then, subtracting this mean from 

 the separate values, he finds the residual for each observation. From these 

 residuals he finds the value of the mean residual for each year, and tabu- 

 lates these "yearly residuals," which thus show the yearly variation in the 

 diameter. 



In the first of these steps, Ambronn was confronted with a difficulty: 

 the series of observations were not strictly homogeneous. In October, 1891, 

 a prism was introduced into the instrument in such a manner that the line 

 joining the centers of the two images could always be brought into the same 



