P/totixjrdpliic Chiirfln;/ of the Heavens. 51 



will be in the year 1900. Each partici[)ating observatory is 

 to provide itself with a twin telescope, eqnutoriallv mounted, 

 one for photograph camera, having an aperture of 3S centi- 

 metres (lo-t inches), and a focal length of 3') centimetres 

 (13 feet), the other of about the same focal length, but of 

 less aperture. The photographic object glasses were to be 

 speciall}^ constructed with curves calculated for the wave 

 lengths near the Fraunhofer line G. The exact form and 

 dimensions of the twin guide telescope were left to the 

 several obsei'vers participating. Gelatine bromide plates, {i\ 

 (lOG millimetres) square to be used (particular make, left 

 also to observers). These plates cover a little over two degrees 

 square at the equator. Two sets of plates to be exposed, one 

 for the catalogue to secure all stars down to the 11th 

 magnitude, the other for the chart to contain all stars down 

 to the 1-ith magnitude. Each set of plates to consist of two 

 series, one to cover every successive 4 square degrees, and 

 the other to cover the junction of four contiguous plates in 

 such a way that its four corners correspond with the centres 

 of these four contiguous plates. Eighteen observatoiies take 

 part in the undertaking — eleven in the Northern, and seven 

 in the Southern Heniisphere. For division of the woik 

 among these, the sky is partitioned into zones, and certain 

 zones given to each observatory in such a way that no 

 observatory will have to work very far from the zenith. 

 Melbourne has the greatest I'ange in this respect, as the 

 zones from (J 5 degrees south to the pole are allotted to our 

 Observaior}^ which gives us a zenith distance of 52 degrees. 

 This was unavoidable, as Melbourne is the most southern of 

 the very few observatories in the Southern Hemisphere. 

 For every plate in each zone, a guide star has to be 

 previously selected, and it was decided by the congi'ess 

 committee that such guide star must be within 22 min. of 

 arc from the centre of the plate to which it belongs. This 

 guide star is used as already described, and its position has 

 to be exactly determined by transit observation to establish 

 a date point to which all stai'S in the photograyjh have to be 

 referred in the final measurements. The determination of 

 these guide stars is in itself a great work, for although the 

 position of a considerable number have already been well 

 determined, and are to be found in existing catalogues, 

 especially in the Northern Hemisphere, there aie still a very 

 large number of plates for which guide stars have to be 

 found, and positions determined. To cover the whole of the 



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