38 



ASTRONOMICAL PROGRESS AND PHENOMENA. 



secretaries; and Dune'r and Tre'pied, assistant 

 secretaries. At the first meeting a committee 

 of nineteen was appointed to consider and re- 

 port upon the size and construction of the in- 

 struments to be employed, and upon the limit 

 of star-magnitudes to be included in the photo- 

 graphs. This committee reported on April 19, 

 and after some discussion it was decided to di- 

 vide the Congress into two sections one to deal 

 with purely astronomical questions, and the 

 other with those involving photography. Each 

 section drew up a series of resolutions, which 

 were further discussed and amended by the 

 Congress in full session, and were finally adopt- 

 ed in the following form: 



1. The progress made in astronomical photography 

 demands that astronomers of the present day should 

 unite in undertaking a description of the heavens by 

 photographic means. 



2. This work shall be carried out at selected 

 stations, and the instruments shall be identical in 

 their essential parts. 



3. The principal objects shall be: 



a. To prepare a general photographic chart of the 

 heavens tor 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 photographic sense to be defined later). 



b. To be able to utilize in the best way, both in the 

 present and in the future, the data obtained by photo- 

 graphic means. 



4. The instruments employed shall be exclusively 

 refractors. 



5. The stars shall be photographed as far as the 

 fourteenth magnitude, inclusive ; this magnitude be- 

 ing indicated provisionally by the scale actually in 

 use in France, and with the reservation that its 

 photographic value shall be definitely fixed afterward. 



6. The aperture of the object-glasses shall be 0-33 

 metre (13'0 inches), and the focal length about 3'43 

 metres (11J feet), so that a minute of arc shall be rep- 

 resented approximately by O'OOl metre. 



7. The directors of observatories shall be at liberty 

 to have the object-glasses made where they desire, 

 provided they fulfill the general conditions laid down 

 by the Congress. 



8. The aplanatism and achromatism of the objectives 

 shall be calculated for radiations near the Fraunhof'er 

 ray G. 



9. All the plates shall be prepared according to the 

 me formula ; this formula to be subsequently agreed 



10. A permanent control of these plates from the 

 point of view of their relative sensibility to the differ- 

 ent radiations shall be instituted. 



11. Questions in regard to the preservation and re- 

 production of the negatives can not at present be set- 

 tled, and shall be referred to a special committee. 



12. The same conclusions are adopted in regard to 

 the photographic magnitudes of the stars. 



13. Eesolution 8 above, in regard to the aplana- 

 tism and achromatism of the object-glasses, shall be 

 understood in the sense that the minimum focal dis- 

 tance shall be that of a ray near G, so as to attain the 

 maximum sensibility of the photographic plates. 



14. The object-glasses shall be constructed in such 

 a manner that the field to be measured shall extend 

 at least 1 from the center. 



15. In order to eliminate fictitious stars, and to avoid 

 inconvenience from minute specks which may exist 

 upon the plates, two series of negatives shall be made 

 for the whole sky. 



16. The two series of negatives shall be so made 

 that the image of a star, situated in the corner of a 

 plate of the first series, shall be found as nearly as 

 possible in the center of a plate of the second series. 



17. Besides the two negatives giving the stars down 

 to the fourteenth magnitude, another series shall be 

 made with shorter exposures, to assure a greater pre- 

 cision in the micrometrical measurement of the funda- 

 mental stars, and render possible the construction of a 

 catalogue. 



18. The supplementary negatives, destined for the 

 construction of the catalogue, shall contain all the 

 stars down to the eleventh magnitude approximately. 

 The Executive Committee shall determine the steps 

 to be taken to insure the fulfillment of this condition. 



19. Each photographic plate to be used in the for- 

 mation of the catalogue snail be accompanied by all 

 the data necessary to obtain the orientation and the 

 value of its scale ; and, as far as possible, these data 

 shall be written on the plate itself. Each plate ol 

 this kind shall show a well-centered copy of a system 

 of cross-wires for the purpose of eliminating errors 

 which may be produced by a subsequent deformation 

 of the photographic film. Further details of this na- 

 ture shall be determined by the Executive Committee. 



20. In the negatives intended for the map, the num- 

 ber of cross-wires to be used in their control and re- 

 duction shall be reduced to a minimum. 



21. The tubes of the photographic instruments shall 

 be constructed of the metal most likely to give an in- 

 variable focal plane, and shall carry a graduation for 

 the determination and regulation of the position of the 

 plate. 



22. The Executive Committee shall choose the ref- 

 erence stars to be used. 



23. The question of the methods of measurement, 

 and the conversion of the numbers obtained into right 

 ascensions and declinations for the equinox of 1900, is 

 left to the Executive Committee. That committee 

 shall first occupy itself with the study and methods of 

 use, of measuring-instruments, giving either rectangu- 

 lar or polar co-ordinates, and based upon the simulta- 

 neous use of scales for the larger distances, and microm- 

 eter screws for scale subdivisions. 



24. The connection of the plates will be effected in 

 conformity with resolution No. 16. 



At the last general session, April 25, the Con- 

 gress elected a permanent committee of elev- 

 en, consisting of Christie, Dun6r, Gill, Paul 

 Henry, Janssen, Loewy, Pickering, Struve, 

 Tacchini, Vogel, and Weiss, and it was decided 

 that all directors of observatories actually 

 taking part in the work of forming the map 

 should, by virtue of that circumstance, become 

 members of this committee. The observa- 

 tories of Algiers, Bordeaux, Buenos Ayres, 

 Paris, Rio de Janeiro, and Toulouse, were an- 

 nounced as now ready to undertake their share 

 in the new work, and Tripled, Rayet, Beuf, 

 Mouchez, Cruls, and Bail! and were thus added 

 to the committee. An Executive Bureau of 

 this committee was elected, consisting of 

 Mouchez, president ; Christie, Dun6r, Janssen, 

 Struve, and Tacchini, members; and Gill, 

 Loewy, and Vogel, secretaries. A special com- 

 mittee was also elected by the Congress to oc- 

 cupy itself with the application of photography 

 to astronomy, other than the construction of 

 the map, having regard to the importance of 

 all these applications, and to the relations that 

 it is desirable to establish between these differ- 

 ent kinds of work. Common and Janssen were 

 requested to take charge of this matter. It is 

 intended that the Executive Committee shall 

 meet every year at one of the observatories en- 

 gaged in the photographic work, and reports 

 of these meetings will be published. 



