POOR, THE FIGURE OF THE SUN 



407 



Table Vn. 



1893. 



Date. 



P.-E. (arc). 



Wt. 



Sept. 8 

 Sept. 9 

 Sept. 11 

 Sept. 20 

 Sept. 21 

 Sept. 27 

 Sept. 28 



— 1".10 

 — 0".94 

 — 0".72 

 — 1".60 

 — 0".76 

 — 0".70 

 — 0".41 



±0''.24 

 ±0".21 

 ±0".18 



±(y'.25 



±0".33 

 ±0".30 

 ±0".29 



2.8 

 3.7 

 5.9 

 2.5 

 1.5 

 1.8 

 1.9 



Forming the mean by weights of the entire series, it is seen that, during 

 the years 1893-94, the equatorial radius was the greater, exceeding the 

 polar by 



P.-E.= — 0".72. 



YERKES PLATES — THE PHOTO-HELIOMETER. 



In the latter part of 1896 a six-inch lens of forty feet focal length was 

 mounted on the tube of the forty-inch Yerkes telescope. This lens was 

 made by Brashear and was specially designed and corrected for photo- 

 graphic work. With it a long series of experiments were carried out and 

 many photographs taken. These experiments were undertaken with the 

 view of determining the best methods of applying photography to the de- 

 termination of the size and shape of the sun. While the photographs 

 obtained can only be regarded as experimental, yet they lead to definite 

 conclusions as to the value of this method and as to its possibilities. 



This investigation of photographic methods led to experiments with a 

 photographic heliometer, and early in 1907 the six-inch was dismounted 

 and a pair of two-inch photographically corrected lenses of twenty-five feet 

 focal length were mounted side by side in the same cell, so as to give over- 

 lapping images of the sun. The cell was arranged so as to revolve about 

 the coUimation axis in a manner similar to the cell of a heliometer. Thus 



