306 SCHLESINGER— PHOTOGRAPHIC DOUBLET [April 24, 



cepted as indicating the size of accidental errors and of certain kinds 

 of systematic error, it cannot be claimed that they tell the whole 

 story. To make certain that star places thus derived are free from 

 serious systematic error of any kind, would require much more obser- 

 vational material and would involve certain extensive intercompari- 

 sons as well as comparisons with star positions derived by wholly 

 different methods. Such a work is contemplated at Allegheny and 

 is in fact well under way. The zone extending from declination 2° 

 north to 2° 10' south is being photographed in duplicate, the centers 

 of one set of plates being upon the eastern or western edges 

 of the other. Each plate will embrace 24 minutes in right ascen- 

 sion or 6° ; the program at the telescope therefore calls for 120 

 plates. To determine the plate constants 602 comparison stars will 

 be employed, an average of ten on a plate. The mean of the two 

 positions for each catalogue star will therefore depend upon about 

 fifteen comparison stars. Thanks to the courtesy and cooperation 

 of Director Campbell and Professor Tucker of the Lick Observatory, 

 the positions of these 602 stars are being determined with the merid- 

 ian circle of that institution. 



Within the limits of declination selected we are measuring the 

 positions of all the stars that appear in the three Gesellschaft zones 

 that overlap. The observing list for the latter was made up of the 

 stars that are designated with (visual) magnitudes 9.0 or brighter in 

 the Bonn Durchmusterung, and as many fainter stars as circum- 

 stances permitted. In our work we shall necessarily omit a few 

 stars that are photographically faint by reason of their color, a 

 few stars that are so bright as to present images too large for ac- 

 curate bisection, and a few doubles that are too close for separation 

 on plates of this small scale. Making allowance for these omissions 

 the completed catalogue will contain about 7,100 stars. It is hoped 

 that this work may not only prove a valuable contribution to our 

 knowledge of the positions and motions of faint stars, but that it 

 may enable astronomers to decide definitely as to the advantages and 

 disadvantages of this form of instrument for the wider applications 

 of the same kind that the future will demand. 



Allegheny Observatory, 



University of Pittsburgh. 



