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THE POPULAR SCIENCE MONTHLY. 



at most of them. If we add those obtained by the French, Germans, 

 and English, the total number available reaches nearly 1,200, accord- 

 ing to the best estimates. 



Fig. 5. 



After the pictures are made and safely brought home, they have 

 next to be measured i. e., the distance (and in the American pictures 

 the direction also) between the centre of Venus and the centre of the 

 sun must be determined in each picture. This is an exceedingly deli- 

 cate and tedious operation, rendered more difficult by the fact that 

 the image of the sun is never truly circular; but, even supposing the 

 instrument to be perfect in all its adjustments, is somewhat distorted 

 by the effect of atmospheric refraction ; so that the true position of 

 the sun's centre with reference to the squares of the reticle is deter- 

 mined only by an intricate calculation from measurements made with 

 a microscopic apparatus on a great number of points suitably chosen 

 on the circumference of the image. The final result of the measure- 

 ment would come out something in this form : Peking, No. 32, time, 

 14 h 08' 20.2" (Greenwich mean time) ; Venus north of sun's centre, 

 735.32"; east of centre, 441.63"; distance from centre of sun, 857.75". 

 (The numbers given are only imaginary.) It is this process of meas- 

 urement which has required so long a time since the transit, and is not 

 yet completed. When it is finished, and the results published in the 

 form indicated, then will come the work of combining all the data thus 

 obtained at all the stations, and from them deducing the true value 

 of the solar parallax. Since, however, another transit is to occur so 

 soon (in 1882), it is not unlikely that astronomers may defer the final 

 grand combination until the observations of that transit also are 

 ready to be included. It is very confidently hoped by most of those 

 who have studied the subject that the remaining uncertainty in the 



