166 Mr. David Gill [June 3, 



Now if a plate is exposed long enough to get satisfactory pictures 

 of stars to the 14th magnitude, the images of the standard stars of the 

 7th, 8th and 9th magnitude will not have the highest perfection, and 

 consequently the places of the fainter stars cannot be measured relative 

 to the ill-defined standard stars with the highest precision. 



This will be evident if we examine actual photographs. 



One illustrates a short exposure, the other a long exposure. The 

 short exposure gives sharj) definition of the brighter stars, the long 

 exposure brings into view a much greater number of stars, but the 

 sharp definition of the brighter stars is completely lost. Therefore, 

 if we wish to have determinations of absolute positions, we cannot 

 have long exposures. 



The meaning of the series of plates of short exposure, and show- 

 ing stars only to the 11th magnitude, is thus explained: 



Of stars to 11th magnitude there are about IJ millions in the 

 sky, and a catalogue containing all these stars may be considered 

 complete for the practical purposes of astronomy, because that magni- 

 tude is the faintest which can be measured with accuracy in the larger 

 class of equatorials usually employed in working observatories. 



I need not enter into detail about the technical means which are 

 to be taken for eliminating the various sources of error, such as con- 

 traction of the photographic film in course of development, and so 

 forth. All these points have been considered by the Congress, 

 or put into the hands of specialists when it appeared that any 

 particular point required further special study, and they are 

 too technical to be entered upon here. The chart of stars to 

 the 14th magnitude will be of importance for many purposes, 

 such as the search for minor planets, and the trans - NejJtunian 

 planet, for variable stars, and for data as to the law of distribution of 

 stars of the higher order of magni-tude. But I do not hesitate to say 

 that the work which astronomers of future generations will be most 

 grateful for, and which will most powerfully conduce to the progress 

 of astronomy, will not be the chart but the catalogue. 



And now. Ladies and Gentlemen, I have dragged you through 

 what I fear has so far been a weary account, to bring you to an 

 apparently very uninteresting conclusion. 



Catalogues and figiu'es are not matters of much popular interest, 

 and yet from such uninviting material has been built ujj the fair 

 structure of the exact astronomy of the present day ; and out of such 

 materials have been evolved the facts which appeal so strongly to 

 the minds of men, and most strongly so because men know that 

 the conclusions rest not on mere imaginings alone, but on solid facts 

 and figures also. 



But now as to the practical execution of this useful work. After 

 all the preliminary details of the operations have been fully dis- 

 cussed — when the instruments have been designed and made, and 

 the mode of working and the methods of measurement and reduction 

 have been devised, the practical execution of the work becomes one 



