164 Mr. David Gill [June 3, 



A preliminary committee having arranged the general order of 

 business, the Congress was opened on the 16th April, and its 

 thoroughly representative character will be understood from the 

 following statement of the nationalities of the members present. 



France .. .. 20 



England and Colonies 8 

 Germany .. .. 6 



Kussia 3 



Holland 3 



U.S.America .. 3 



Austria 2 



Sweden 2 



Denmark .. .. 2 



Belgium 1 



Italy 1 



Spain 1 



Switzerland .. .. 1 



Portugal .. .. 1 



Brazil 1 



Argentine Kepublic 1 



Before the Conference, a great many people, I will not say 

 astronomers, held that the chief object was to photograph as many 

 stars as possible, and simply preserve these plates or issue photo- 

 graphic copies of them, so that astronomers of the future, by merely 

 comparing one of these originals or copies with a similar photograph 

 of the same part of the sky taken 50 or 100 years hence, would find 

 out what stars had changed in position or magnitude, or whether any 

 new star had appeared. 



There is no doubt this was the view of the popular writers — it is 

 very easily understood, and it appeals very directly to the imagina- 

 tion. Such a project alone would no doubt have had great impor- 

 tance and would probably in the future have brought to light a great 

 many very interesting isolated facts. 



But for the broader and more refined purposes of astronomy, for 

 the discussion of such great questions as the motion of the solar 

 system in space, the common movement of large groups of stars, 

 the accurate determination of precession, and the general refinement 

 of astronomy of precision, these mere pictures would have no value. 



It was essential for these larger and more permanently important 

 ends that all data should be provided for the most refined deter- 

 mination of the absolute position of any star upon any plate. This 

 view was endorsed by the Congress. 



The objects of the survey of the heavens to be carried out were 

 defined ultimately thus : — " To make a photograj^hic chart of the sky 

 for the present epoch, and to obtain the data for determining the 

 positions and magnitudes of all the stars to the 14th magnitude," as 

 that magnitude is at present defined in France. 



At present there are no exact determinations of stellar magnitude 

 to that order of faintness, and the considerations which really guided 

 the Conference were, that stars which are called 14th magnitude are 

 photographed by the Henrys with an exposure of about 15 minutes of 

 time. With such an exposure the time required for the work con- 

 templated by the Congress would not be too great, but to demand 

 long exposures would lead to the loss of many plates by interruptions 

 from clouds, &c., and would unduly prolong the time required for 

 completion of the whole work. As it is, the number of stars photo- 

 graphed to 14th magnitude will number about 20 millions. 



It was seriously urged that stars to the 15th or even 16th 



