472 PHOTOCxRAPHY IN THE SERVICE OF ASTRONOMY. 



striicted, by way of trial, an objective of 0.10'", wbi(;b gave very good 

 results, tbe Heury brotbers undertook to execute tbe optical ijart of a 

 definitive apparatus of 0.33'" in aperture, for wbich Mr. Gautier was to 

 farnisb tbe mecbanical part. Tbe new instrument was mounted at tbe 

 observatory in 1885 and bas not ceased since tben to be in active use. 

 The sensitiveness of tbe plates is such that tbe image of a star of tbe 

 first magnitude is obtained in less Iban a bundredtb of a second ; tbat 

 of a star of tbe sixtb magnitude in a balf second ; for tbe tentb magni- 

 tude tbe lengtb of exposure is about twenty seconds; for tbe fifteentb 

 about tbirty-tbree minutes ; for tbe sixteentb one hour and twenty min- 

 utes is necessary.* Tbe stars of tbe sixteentb magnitude! Here we 

 are already far beyond tbe limits of visibility for tbe best telescopes under 

 tbe sky of Paris ! " Says Moucbez, even stars of tbe seventeenth magni- 

 tude have been certainly obtained, which without doubt have never 

 before been seen." Finally tbe Paris plates have revealed tbe exist- 

 ence of nebube hitherto unknown in the regions which bad been often 

 explored with tbe aid of the most jjowerful instruments; such is the 

 nebula of Maia, in tbe Pleiades, whose presence has been since di- 

 rectly verified. 



After such success we understand that the director of the observa- 

 tory has not hesitated in taking the initiative in an international agree- 

 ment on the subject of tbe execution of the complete chart of the sky, 

 by the means of photography. The possibility of this considerable 

 work being to-day fully demonstrated, be bas said to astronomers, we 

 have assumed, for the science of the future, tbe trust of going about it 

 without delay; whatever may be the value of the works in course of 

 execution in the various observatories, they will never have, for the 

 astronomers of future ages, an importance comparable with that of this 

 general inventory wbich we shall be able to bequeath to them. It is 

 besides, indispensable for us to concert together and distribute the labor 

 and conclude ujion a plan of work, in order to avoid the waste of force, 

 the gaps, and the useless repetitions, and tbe result will be a work 

 truly homogeneous. In regard to tbe expense which the enterprise . 

 will involve, it will be without doubt large enough necessarily, but 

 very inconsiderable relatively to the importance of the result. 



The astro-photographic congress convened in Paris, as we have said, 

 in tbe month of April, 1887; sixteen nations were there represented. 

 At the commencement certain technical questions were settled; the 

 employment of retlecting telescopes, in spite of the advantages wbich 

 they offer in some connections, has been rejected for the execution of 

 tbe chart of the sky, and a unanimous vote recommended refracting 

 telescopes; they will be constructed vsimilar to the photographic tele- 



* We take this iuforrnation from the notice of Admiral Mouchez, which dates from 

 1887, but these times of exposure are already much shortened by employing luore 

 sensitive plates, such as the American plates which Pickering uses, and which the 

 Henrys have tested in their turn. 



