86 president's address — section a. 



hope to show you presently that 1 have the design ready l)y which 

 meridian transit work will be done by photography in a far more 

 exact way than it can be done by the eye. 



Professor Pritchard, of Oxford'^% was the first to apply the photo- 

 graphic method to the determination of stellar parallax. He con- 

 ceived the idea in May, 1886, put it to the test of experiment by 

 determining the parallax of 61 Cygni, not with the object of deter- 

 mining the distance of a star so well known, but for the purpose of 

 putting his novel method to a crucial test. He selected a star the 

 parallax of which had been so well determined that there was a 

 definite value before he began ; probably the parallax of this star 

 was better established than that of any other. His great success 

 is well known, and the accuracy of the method so great, tliat a most 

 satisfactory value of the parallax was obtained coming close to the 

 mean of the four best values previously determined by older 

 methods. 



The Oxford value is : — 



6V Cygni 0-4.38 



61 2 Cygni 0-441 



Anwer's, value 0-348 



BesseTs, value 0--564 



Ball, value 0-468 



Asap Hall, value 0-261 



Mean 0-410 



The professor determined the parallax of thirty stars conveniently 

 situated from Oxford of first and second magnitudes; and collecting 

 those determined by (Jill and Elkin of stars of the first magnitude, 

 he was able to give in his "Researches into the History of Stellar 

 Parallax" a list of ninety-three bright stars, the distances of which 

 have been recently measured. This list includes the majority of 

 the bright stars, and from this he deduced that the averajje 

 parallax of first magnitude stars is 0-89" and of second magnitude 

 0"056''. There are considerable deviations from the mean in both 

 classes ; but the fact remains that the first magnitude stars are 

 nearer to us than the second, and both very much nearer than the 

 faint stars with which they Avere compared to determine their 

 distances. 



On October 24th, 1886, Dr. Isaac Roberts, who had been so 

 successful in photographing faint objects, turned his telescojje on 

 the nebula about Mia, and found that it was much more extensive 

 than had been supposed ; many branchings seemed to form a back- 

 ground for the whole cluster of the Pleiades. 



In April, 1887, a conference of fifty-four astronomers from all 

 parts of the world met at Paris, and agreed upon a scheme in which 

 eighteen of them undertook to carry out the work. All were to 

 use star cameras of the same size and focal length and take two 



