84 president's address — section a. 



(1883) taken up by Professor Pickering at Harvard College, and 

 Mrs. Draper was induced to provide the money for this work as a 

 memorial to her husband — one of the noblest monuments ever 

 raised over a scientific man. 



In May, 1884, MM. Henry Bi'others were making photographic 

 experiments to test the accuracy of their method of measuring 

 double stars from ])hotographs, and in September the same year 

 they had succeeded in photographing the small stars of the ecliptic. 

 The difficulty of recording the positions of these stars in the 

 old laborious way had induced them to try to photograph this 

 part of the heavens in order to avoid the labor of lecording 

 them by the eye and hand. The method when completed not 

 only recorded the stars which were required in the search for 

 small planets, but actually made it unnecessary to look for the 

 planets through a telescope, because they show themselves amongst 

 the stars by making a trail instead of a round spot, and this was 

 done with the experimental G^in. star camera. This success was 

 so satisfactory that they began at once to make an objective of 

 13|^in. for this special purpose, and expected to be able to photo- 

 graph stars to the twelfth magnitude. 



With this larger star camera, on November 16th. 1885, they 

 found in taking photographs of the Pleiades a hitherto unknown 

 nebula about the star Mia. The star camera had literally called 

 it from darkness to light. 



In October, 1884, MM. Henry'^ had got the new 13in. star camera 

 fairly at work. They had taken a photograph of the cluster of 

 stars in Hercules, giving it fifty minutes' exposure, and found 550 

 stars of from seventh to twelfth magnitude In another place with 

 sixty minutes' exposure on a surface five degrees square they counted 

 2,790 stars between sixth and fourteenth magnitades, and traces of 

 fifteenth magnitmie stars, whose diameter was only — o-ooiiiM and 

 Admiral Mouchez, in referring to this work, said they had gone so 

 far as to secure images of a few stars of seventeenth magnitude, 

 "and such stars, without doubt, have never been seen before" — they 

 are beyond the reach of any telescope. 



Their experiments proved that they could photograph a star of 

 the first magnitude in o^^oth part of a second, one of fifth in one- 

 fifth of a second, one of sixth in half a second, one of tenth in 

 fifty seconds, and stars of sixteenth magnitude, only just visible 

 in the largest telescopes, in eighty-three minutes, and their 

 experiments led them to estimate the whole number of stars 

 visible in Sir John Herschel's telescojje which they could 

 photograj)h as twenty-two and a half millions. Herschel, as the 

 result of niMuy counts in various parts of the sky, had estimated 

 the number he could have seen in the whole sky, if he spent 

 forty-five years in doing it, as twenty and a half millions. 



It is obvious that a photograph taken now and showing 

 accurately the positions of the stars will, if compared by super- 



