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8CIENCE- 



[N. S. Vol. XXV. No. 637 



fourteenth, magnitude would appear on plates, 

 having exposures of ten minutes, taken with 

 the twenty-four-inch Bruce telescope, and on 

 plates, having exposures of an hour, taken 

 with the eight-inch telescopes. The average 

 number of times the whole sky is thus cov- 

 ered does not exceed three or four. A large 

 part of the southern sky is covered by plates 

 having an exposure of an hour or more, taken 

 with the twenty-four-inch Bruce telescope. 

 One hundred and sixty regions, covering a 

 tenth of the sky, have exposures of four hours. 

 While, therefore, material exists for studying 

 the variability of stars of the thirteenth mag- 

 nitude or brighter, but few images can be 

 found of stars much fainter than this limit, 

 except in special regions like the Nebula of 

 Orion. This limit could probably be extended 

 nearly two magnitudes, if the following plan 

 could be carried out. For the present it seems 

 best to confine this work to photographs hav- 

 ing an exposure of half an hour or more, and 

 taken with doublets whose aperture is six 

 inches or more. Photographs taken with large 

 reflectors or single lenses may take fainter 

 stars, but in general they relate to special re- 

 gions and except in the charts of the Astro- 

 graphic Congress do not cover large parts 

 of the sky. The latter great work will of 

 course, eventually, furnish one or two images 

 of all stars of the fourteenth magnitude, and 

 brighter. 



Information is frequently desired regarding 

 very faint stars. For instance, evidence of 

 the previous existence of a new star; of a star 

 now missing; the position of an asteroid; the 

 magnitude at minimum of a faint variable; 

 early position of a faint star suspected of 

 proper motion. Three questions suggest them- 

 selves. Where are the principal collections of 

 photographs showing faint stars, and what is 

 their character and extent? Does a photo- 

 graph exist which is likely to contain the 

 required information? Is this photograph 

 accessible, so that the information desired can 

 be secured from it? Satisfactory answers to 

 these questions can generally be obtained for 

 stars brighter than the thirteenth magnitude, 

 but not for fainter stars. Astronomers hav- 



ing collections of photographs showing such 

 faint stars are urged to publish the material 

 required to answer these questions, or to send 

 the information needed to this observatory. 

 The principal facts desired are given below: 



1. A description of the instrument, stating 

 its location; form of lens; maker; aperture; 

 focal length, or scale of photograph, that is, 

 number of seconds of arc to the millimeter; 

 size of plate, or region covered; diameter of 

 circle over which stars as faint as the four- 

 teenth magnitude are shown; kind of plate 

 used; name of observer. 



2. A list of photographs, which should in- 

 clude the number designating each plate; the 

 time of the center of the exposure (found by 

 adding half the duration of the exposure to 

 the time of beginning), expressed either in 

 Julian Days and thousandths following 

 Greenwich Mean Noon, or in calendar days, 

 hours and minutes; the right ascension and 

 declination for 1900 of the center of the plate ; 

 the time of exposure in minutes; the quality 

 on a scale of 5, 5 denoting that the images 

 are circular and that no serious defect ap- 

 pears upon the plate, 4 that the images are 

 nearly circular so that their positions can be 

 accurately measured, 3 that faint stars are 

 shown so that the relative brightness can be 

 accurately estimated, even if the images are 

 not circular. 



3. A statement whether the owner is willing 

 to examine and if necessary measure his pho- 

 tographs to furnish any desired information; 

 to lend them under suitable restrictions, so 

 that copies may be made; to furnish contact 

 prints at cost. 



It is the policy of the Harvard Observatory 

 to make the material it has collected as widely 

 useful as possible. Accordingly, preparations 

 are being made to publish the facts asked for 

 in Nos. 1 and 2. For many years the observa- 

 tory has offered to furnish copies of its photo- 

 graphs at cost to whoever might desire them. 

 If this policy is adopted at other observatories, 

 a purchase of large numbers of photographs 

 of faint stars is contemplated here. The ob- 

 servatory will take charge, temporarily or per- 

 manently, of good photographs for which the 



