POOR, THE FIGURE OF THE SUN 415 



In 1893-94 the plates by Wilson give a result having the same sign as 

 Ambronn's mean for these same years. The photographic result, however, 

 is very much larger. The photo-heliometer promises to give accurate 

 results; and a forty-foot instrument of this character should finally decide 

 the question as to the shape of the sun. 



The only conclusion that can safely be dravin from the conflicting data 

 is, that the exact shape of the sun is not known with certainty. On the 

 average, it approximates very closely to a sphere; the difference between 

 the equatorial and polar radii, if such difference exists, being probably not 

 more than 0".25. The heliometer measures show marked variations in 

 the different years, and a part of the conflict in the results may be due to 

 actual variations in the shape and size of the sun. 



VAEIABILITY OF THE SUN's DIAMETER. 



The following table exhibits the results of the various investigations 

 mentioned in the first part of this paper. 



Table XH. 

 Meridian Observations. 



1809 Lindenau Periodic variations. 



1809 Lindertau Re-discussed 1889 by Auwers — no variations. 



1871 Secchi Varies inversely with the number of sun-spots. 



1871 Secchi Re-discussed 1885 by Auwers — no variations. 



1874 Ncwcomb & Holden . . No long-term variation. 



1895 Auiccrs 1st discussion — varies with number of sun-spots. 



1895 Auwers 2d discussion — observed variation due to variable 



personal equations. 



Heliometer Observations, 



1905 Ambronn Periodic variation of 0".l, but no relation to sun- 

 spot period. 



These investigations dealt with possible variations in either the equa- 

 torial or polar diameter, as in Auwers' work, or in the average or mean 

 diameter, as in Ambronn's paper. But the state of the atmosphere, the 

 sharpness of the image, the color of the shade-glass used, — all these 

 affect the measured diameter, and introduce accidental and semi-periodic 

 errors. Any actual variations in the diameter will, therefore, be masked by 

 these errors of observation, and correspondingly difficult to determine. On 

 the other hand, if there be a variation in the sun's diameter, it is improbable 

 that such variation affects both the polar and equatorial diameters in the 

 same way. The measured differences between the two diameters is thus 

 more likely to show the presence of a variation than vn.\\ the direct measures 



