158 CARNEGIE INSTITUTION 



tudes of 454,875 stars, nearly all south of declination — 18°, as 

 measured by Kapteyn on Gill's plates. This great work is memo- 

 rable not only as the first of its kind, but for the promptness with 

 which it has been completed and published. The successful results 

 of the MM. Henry at the Paris Observatory led to the formation of 

 a permanent international committee for making a photographic 

 chait of the sky. At the first international congress held in Paris 

 in 1887, seventeen different nationalities were represented. A gi- 

 gantic plan was formed in which eighteen observatories took part. 

 Each agreed to have a 13-inch telescope constructed, and to photo- 

 graph a certain zone with a short and a long exposure. A catalogue 

 was to be formed from measures of all the stars shown on the plates 

 taken with short exposures, giving about a million and a quarter 

 stars to the eleventh magnitude. The long exposure plates, con- 

 taining perhaps twenty million stars to the fourteenth magnitude, 

 were to be published in the form of charts. The Potsdam Observa- 

 tory has published two quarto volumes of about 500 pages each, 

 giving the results of measurements of 57 and 38 plates. They give 

 the precise rectangular coordinates of each star, but, owing to the 

 great expense of the reductions and the lack of suitable meridian 

 circle positions, the values of the right ascension and declination are 

 only approximate. Three hundred of the charts have been pub- 

 lished. The size is 28 x 28 centimeters and the scale 30 seconds to 

 one millimeter. Eleven thousand of these plates would be required 

 to cover the whole sky. Although fifteen years have elapsed since 

 the first conference, the amount of material published by no means 

 represents the amount of work done. No zone has been taken by 

 any observatory in the United States. Including the very heavy cost 

 of measurement and reduction, it is doubtful if the entire work could 

 be completed at a total expenditure of $3,000,000. This great un- 

 dertaking, if ever completed, will furnish a record of the state of the 

 heavens upon which the work of centuries may be based. The plan 

 adopted at the Harvard Observatory in the early eighties recom-, and 

 mended to the first Paris conference in 1887, was that all the photo- 

 graphs should be taken with a single photographic doublet. Each 

 plate taken with the Bruce 24-inch telescope, completed in 1892, 

 which is of this type, covers a field of 25 square degrees, as com- 

 pared with the four square degrees covered by the international 

 charting telescopes. The errors in such plates due to distortion 

 have been shown by Professor Turner to be insensible. The entire 

 sky in either hemisphere can be charted with this instrument in one 



