6 ANNIVERSARY ADDRESS, _ 
discussion which has been going on in Europe and America, has 
thrown much light upon the question at issue ; and within thelast 
few months (February, 1882) statements have been made as to the 
accuracy with which photos. can be measured, which may 
materially alter the intentions of those charged with arranging the 
work of observing the coming transit, Professor Pritchard has pub 
lished in the Observatory for February, 1882, his experience astd 
the “admissibility of photography among other means of accurate 
measurement of celestial phenomena.” ‘The results of his exper 
ments made at the Oxford Observatory, in measuring the diameter 
of the moon, are as follows :—Seven photos. were taken and the 
extreme difference between the values of the moon’s diameter 
derived from these was only 0-71”, and the photos. are so small 
that one second of are is only 1-7000th of an inch (006), and the 
probable error of determining the position of any point on the 
photos. is only 0:35, and he remarks : “When such are the resulis_ 
of the Oxford lunar photos. and the American solar ones, it sels 
to me a matter of regret that the International Committe) 
assembled in Paris recently, determined not to adopt the phot 
graphic method in European expeditions. And in a paper 
D. P. Todd, assistant in the office of the American Nauticl 
Almanac, published in June, 1881, he discusses the value of be 
American photographs as a means of determining the solar pa all a ; 
and arrives at the conclusion that the probable error of @ singe 
photograph is only 0°88”, and the probable error of the para 
derived from the whole number (213) is 0-034”, and the result . 
parallax is 8-883", : 
The same photographic instruments which were used by® i 
American observers of the transit of Venus were used again + 
was only 0:553”, or consi dlewably lone. thas: :thint Seentll for ¢ 
photos. of the transit of Venus, As the photos. may be tak 
oD 
