ASTRONOMY. 145 



reported on Ain-il 19, and after some discussion it was decided to divide 

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

 questions, and the other with questions pertaining to the photographic 

 side of the problem. Each section drew np 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 

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

 convert the positions of the stars upon these last plates into the usual 

 CO ordinates of right ascension and declination, and to publish them in 

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

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

 servatories or nations. 

 Following are the resolutions in detail as finally adopted: 



(I) The progress made in astronomical i)hotography demands tiiat 

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

 tion of the heavens by photograhpic means. 



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

 ments shall be identical in their essential parts. 



(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 possible accuracy 

 (magnitudes being understood in a ])hotographic sense to be defined 

 later), b. To be able to utilize in the best way, botli in the present and 

 in the future, the data obtained by i)hotographic means. 



(4) The instruments employed shall be exclusively refractors. 



(5) The stars shall be photographed as far as the fourteentii nuigni- 

 tude, inclusive ; this magnitude being indicated provisionally by the 

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

 tographic value shall be definitely fixed afterward. 



(G) The aperture of the object-glasses sliall be 0.33 meter (13. 

 inches), and the focal length about 3.13 meters (11^ feet), so that a 

 minute of arc shall be represented approximately by 0.001 meter. 



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

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

 ditions laid down by the ('ongress. 



(8) The aplanatism and achromatism of the objectives shall be calcu- 

 lated for radiations near the I'raunhofer ray G. 



(0) All the plates shall be prepared according to the same formula ; 

 this formula to be subseipiently agreed upon. 



(1(>) A i)ermanent control of tliese i)Iates from the jxiint of view of 

 their relative sensibility to the dilferent radiations shall be instituted. 



(I I) Questions iu regard to the preservation and reproduetiou of the 

 lu^gatives can not at present be settled, and shall l)e referred to a 

 8pe(;ial committee. 



(lli) The same conclusions are adopted in regard to the photograi»hic 

 magnitudes of the stars. 



(13) llesolution 8 above, in regard to the ajilanatism and achromatism 

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

 mum foeal distance shall be that of a ray near (r, so as to attain the 

 maximum sensibility of the photographic plates. 

 H. Mis. U2 ^10 



