ASTRONOMY. 145 



Imported on April 19, and after some discussion it was decided to divide 

 the Congress into two sections — one to deal with purely astronomical 

 questions, and the other with questions pertainiufi^ to the pliotographic 

 side of the problem. Each section drew up a series of resolutions which 

 were further discussed and amended by the Congress in full session, the 

 result being an agreement to adopt refracting telescopes of 13 inches 

 aperture and 11^ feet focal length, and to undertake two series of pho- 

 tographs of the entire sky, one taking in stars as faint as the fourteenth 

 magnitude, the other stopping at the eleventh. The ultimate aim is to 

 convert the positions of the stars upon these last i^lates into the usual 

 co-ordiriates of right ascension and declination, and to publish them in 

 the form of a star catalogue, and to prepare from the series embracing 

 the fainter stars a set of glass positives for each of the cooperating ob- 

 servatories or nations. 

 Following are the resolutions in detail as liually adopted: 



(1) The progress made in astronomical photography demands tiiat 

 astronomers of the present day sliould unite in undertaking a descrip- 

 tion of the heavens by photogra-hpic means. 



('^) This work shall be carried out at selected stations, and tlie instru- 

 ments shall be identical in their essential i)arts. 



(3) The principal objects shall be : a. To prepare a general photo- 

 graphic chart of the heavens for the present epoch, and to obtain data 

 which shall enable us to determine the positions and magnitudes of all 

 stars down to a certain magnitude, with the greatest i)Ossible accuracy 

 (magnitudes being understood in a photographic sense to be detlned 

 later), h. To be able to utilize in the best way, both in the present and 

 in the future, the data obtained by photograpliic means. 



(I) The instrumiuits employed sliail be exclusively refractors. 



(5) The stars shall be pliotographed as far as the fourteenth magni- 

 tude, incbisive ; this magnitude being indicated i>rovisionally by the 

 scale actually in use in France, and with the reservation that its pho- 

 tographic value shall be definitely fixed afterward. 



{(')) The a[)erture of the object-glasses shall be 0.33 meter (13. 

 inches), and the focal length about 3.13 meters (llj feet), so that a 

 minute of ar» shall be represented approximately by 0.001 meter. 



(7) The directors of observatories shall be at liberty to have the object- 

 glasses made where they desire, provided the.y fulfill the general con- 

 ditions laid down by the (Congress. 



(S) The ai)lanatism and achromatism of the objectives shall be calcu- 

 lated for radiations near tlie Fratinhofer ray G. 



{{)) All the plates shall be prei)ared according to the same formula; 

 this formula to be subsecjuently agreed uj)on. 



(10) A i)ermanent e-ontrol of tliese ])la,tes from tlu^ ])oint of view of 

 their relative sensibility to tlu^ dirfei'ent radiations shall be instituted. 



(II) (^iiestii^ns in regard to the i)reservati()n and reprodiuttion of the 

 negatives can not at present be settled, and shall be referred to a 

 si)ecial committee. 



(1-5) The same conclusions are adojjted in regard to the photograi)hic 

 magnitudes of the stars. 



(13) Jiesolution 8 above, in regard to the aplanatism and achronuitism 

 of the object-glasses, shall be understood in the sense that the mini- 

 nuim focal distance shall be that of a ray near (I, so as to attain the 

 maximum sensibility of the photograpliic plates. 

 H. Mis. U2 10 



