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ANNALS NEW YORK ACADEMY OF SCIENCES 



the line joining the centers of the two lenses could readily be placed parallel 

 to the equatorial or to the polar diameter of the sun. The apparatus was 

 attached to the great forty-inch Yerkes telescope and a series of photographs 

 taken by Mr. Fox. On each day a set of two photographs was taken, one 

 of the polar and one of the equatorial diameters. As a rule these two 

 exposures were made within three or four minutes of each other. 



On each photograph the length of the chord common to the two images 

 is a measure of the radius passing through the extremity of the chord. This 

 chord furnishes an accurate measure of the radius, for slight changes in 

 the radius produce large changes in the chord. A given change, A r, in the 

 radius will produce a change, A d, in chord approximately equal to 



Ad= ^ Ar 

 d 



where r and d are the respective lengths of the radius and the chord. If, 

 therefore, the chord be one-third the radius, then a change of only 0".3 in 

 the radius will produce a change of 1" in the chord. As the scale of the 

 photographs is such that one millimeter is approximately equal to 27", 

 changes of O'MO in the radius would under these circumstances produce 

 changes of 0.01 mm. in the chord; and on good photographs quantities of 

 this size can be readily measured. 



On the test photographs made at Yerkes, the image at distance unity 

 was 71 mm. in diameter and the chord 32 mm. long. When, therefore, 

 the line joining the centers of the lenses was parallel to the equator of the 

 sun, the radii passing through the extremities of the chord were inclined 

 26° to the equator; and similarly, in the reversed position of the objective, 

 the measured radii made angles of 26° with the axis of the sun. In other 

 words, the radii as measured on the equatorial and polar plates cut the 

 surface of the sun in latitudes 26° and 64° respectively. The results re- 

 duced to distance unity for these test plates are shown below. 



Table VHI. 



Means 



35.505 — 0".95 



