PHOTOGRAPHY IN THE SERVICE OF ASTRONOMY 473 



scope of the Paris observatory. In regard to tlie limit of tlie ina<;ni- 

 tiides of the stars to be pbotograplied tbere was some difliinilty in com- 

 ing to an agreement. Taking into consi(Un'ation the notable difference 

 in the length of exposnre necessary for bright stars and very faint 

 stars, it was finally decided to make two classes of plates designed for 

 two different nses. 



For the donble series of plates devoted to the picture of the sky, 

 which is to comprise the stars to the fourteenth magnitude, the length 

 of the exposure will be (in tlie climate of Paris at least) in the vicinity 

 of twelve minutes.* For the supplementary series of plates comprising 

 the stars to the eleventh magnitude only, and which must, on the one 

 hand, secure an extreme precision in the micrometrical measurement of 

 the stars of reference, and on the other hand, furnish the elements for 

 a catalogue, the length of exposure will be much shorter (about thirty- 

 five to forty seconds). This catalogue will probably contain one and 

 one-half million stars, — more than double the number that are cer- 

 tainly known to day. In regard to the number of stars which will be 

 found represented upon the chart properly so called, it may be esti- 

 mated to be from ten to fifteen millions. The two series of plates which 

 will serve for constructing the chart will be arranged in such a wixy 

 that the image of a star, situated in the corner of one plate of the first 

 series, will be found as near as possible in the center of a plate of the 

 second series ; it is hoped that this will suffice for eliminating false 

 stars and remove the inconvenience of unsensitized points which must 

 exist on the ]>lates. 



In adopting for the chart an exposure of thirty minutes, it would be 

 possible to reach the fifteenth magnitude and obtain a double or triple 

 number of stars, perhaps thirty or forty millions, and i)ossib]y more. 

 This was what several members of the congress desired, who could only 

 with reluctance decide to curtail thus the common work of the astron- 

 omers of the nineteenth century. Monchez, notably, has made the re- 

 mark that the limit to which we are confined is very near that of the 

 asteroids whicli are discovered every day ; to obtain appreciable traces 

 of these small stars, the exposure of twelve minutes runs the risk of 

 being insufilicient. Those who have combated the extension of the sur- 

 vey beyond the fourteenth magnitude have ])loaded, in the first place, 

 the length of time which the comi)letioii of the (;hart, under these con- 

 ditions, would demand. In rejdy to them it mny be said that in fixing 

 at 14,000 1 the total number of plates necessary for the execution of the 

 chart, and in supposing that the work will be distributed among fifteen 

 or twenty observatories each observatory will have only 1,000 plates 

 to furnish; in counting twelve minutes to each plate, the work would 

 easily be accomplished in one or two years ; four years would suffice, in 



'Perbapa also much less with the more sensitive plates. 



t III counting C> square degrees to a plate, 7,000 will be needed to cover the sky, and 

 14,000 with the duplicates. 



