ADDRESS. 33 



of the comet of 1882 taken at the Cape Observatory under the superin- 

 tendence of Dr. Gill, and the remarkable star charts of the Brothers 

 Henry which followed two years later, astonished the astronomical world. 

 The great excellence of these photographs, which was due mainly to the 

 superiority of the gelatine plate, suggested to these astronomers a complete 

 map of the sky, and a little later gave birth in the minds of the Paris 

 astronomers to the grand enterprise of an International Chart of the 

 Heavens. The actual beginning of tlTe work this year is in no small 

 degree due to the great energy and tact with which the Director of the 

 Paris Observatory has conducted the initial steps, through the many 

 delicate and difficult questions which have unavoidably presented them- 

 selves in an undertaking which depends upon the harmonious working in 

 common of many nationalities, and of no fewer than eighteen observa- 

 tories in all parts of the world. The three years since 1887 have not 

 been too long for the detailed organisation of this work, which has 

 called for several elaborate preliminary investigations on special points 

 in which our knowledge was insufficient, and which have been ably 

 carried out by Professors Vogel and Bakhuyzen, Dr. Trepied, Dr. Scheiner, 

 Dr. Gill, the Astronomer Royal, and others. Time also was required for 

 the construction of the new and special instruments. 



The decisions of the Conference in their final form provide for the 

 construction of a great photographic chart of the heavens with exposures 

 corresponding to forty minutes' exposure at Paris, which it is expected 

 win reach down to stars of about the fourteenth magnitude. As each 

 plate is to be limited to four square degrees, and as each star, to 

 avoid possible errors, is to appear on two plates, over 22,000 photographs 

 will be required. For the more accurate determination of the positions 

 of the stars, a reseau with lines at distances of 5 mm. apart is to be 

 previously impressed by a faint light upon the plate, so that the image 

 of the reseau will appear together with the images of the stars when the 

 plate is developed. This great work will be divided, according to their 

 latitudes, among eighteen observatories provided with similar instru- 

 ments, though not necessarily constructed by the same maker. Those 

 in the British dominions and at Tacubaya have been constructed by Sir 

 Howard Grubb. 



Besides the plates to form the great chart, a second set of plates for a 

 catalogue is to be taken, with a shorter exposure, which will give stars to 

 the eleventh magnitude only. These plates, by a recent decision of the 

 Permanent Committee, are to be pushed on as actively as possible, though 

 as far as may be practicable plates for the chart are to be taken concur- 

 rently. Photographing the plates for the catalogue is but the first step 

 in this work, and only supplies the data for the elaborate measurements 

 which have to be made, which are, however, less laborious than would 

 be required for a similar catalogue without the aid of photography. 



Already Dr. Gill has nearly brought to conclusion, with the assistance 



1891. D 



