INTERNATIONAL SCIENCE. 497 



by the guaranties of the different countries which, as already men- 

 tioned, take the form of subscriptions for copies of the catalogue, 

 so that it may be said that the central office is self-supporting. 

 After so short a time of Avorking, this success must be a source of 

 considerable satisfaction to Professor Armstrong and those who have 

 helped to initiate the Avork. But the expenses' incurred in London 

 only represent a fraction of the total cost of the work. Most of the 

 countries establish regional bureaus which prepare the slips and for- 

 ward them to London. This really constitutes the most serious part 

 of the Avork. In Germany, for instance, the regional bureaus are 

 under Professor Uhlworm, one of the university librarians, who is 

 helped by six assistants and devotes his whole time to the work. 



I pass on to an undertaking of a very different kind, but still one 

 Avhich must be included in the class which primarily aims at cata- 

 loguing. The accurate determination of the positions of the stars for 

 a particular period is a Avork which must precede all exact measure- 

 ments of their proper motions. Hence it constitutes a fundamental 

 problem of astronomy. The multitude of stars seen on a bright 

 night is bewildering to the casual obserA'er. They are described in 

 poetical Avritings as innumerable, but Avhen an actual count is made it 

 is found that their number is really moderate, and it is doubtful if 

 more than 2,000 stars haA'e eA'er been visible to the naked eye at the 

 same time. The use of the telescope considerably increases this 

 number, according to the size of the object glass or reflecting mirror 

 used. Thus, Argelander in his great star catalogue included nearly 

 324,200 stars Avhich he observed through his telescope of 4 inches 

 aperture. The advent of photography, and the manufacture of suit- 

 able lenses to be used in connection Avith photography, increased the 

 astronomical output of a fine night to such an extent that it became 

 possible to make a further and very substantial advance. The inter- 

 national star catalogue, Avhich is at present being constructed, owes 

 its origin chiefly to the hard work of Admiral Mouchez, who was at 

 the time director of the Paris Observatory, and who became converted 

 to the feasibility of the plan by the excellent results obtained by the 

 brothers Henry, the pioneers in star photography. He was assisted 

 by the energetic support of Sir David Gill, to whom the first sug- 

 gestion Avas due. The programme of Avork was determined upon at 

 an international conference Avhich met at Paris in the year 1887. 

 Eighteen observatories were to take part in the work, the telescopes to 

 be used were to haA^e an aperture of 13 inches, and such a focal length 

 that a millimeter on the plate corresponded to one minute of arc. 

 Each observatory had a certain region of the sky assigned to it, and 

 undertook to coA'er this region four times, twice with plates of short 

 exposure, twice with plates of long exposure, and to measure all the 



SM 1906 32 



