1887.] on the Ajjplicaiions of Photography in Astronomy. 165 



magnitude and higher should be photographed, but it was felt that 

 there was real danger of failure in an attempt to do too much. 



It no doubt produces a strong effect on the imagination to be told 

 that astronomers are to be engaged on making charts of the sky 

 which will contain 60 or 100 millions of stars, or photographing 

 stars on their plates which cannot be seen at all in the most powerful 

 telescopes. There is thus a strong temptation to yield to this demand 

 for sensation, to produce a few astonishing plates with the loss of 

 much precious time, and to sacrifice the real progress of astronomy 

 to the love of the marvellous. Besides, what are you to do with 

 pictures of 100 millions of stars when you have got them? What 

 would be the use of pictures of all these stars, unless at some 

 future time a sufficient number of astronomers were to arise to 

 compare similar photographs, taken, say one hundred years 

 hence, wdth the photographs taken in our day ? I am happy 

 to think that the number of men who devote themselves to the 

 pursuit of astronomy is on the increase, but I have no desire that 

 the number of men in Great Britain who occupy themselves exclu- 

 sively with astronomy will ever correspond with that in the floating 

 island of LajDuta, as described by Dean Swift, where all the men 

 were exclusively occupied with astronomy, and had to be flapped on 

 the head with little bladders containing parched peas to arouse them 

 from their abstract occupations. And yet, unless something of this 

 sort happens, I see no adequate prospect of the utilisation of pictures 

 of 100 millions of stars. 



The Congress, therefore, very widely limited their chart plates to 

 the 14th magnitude. But, as was well said by M. Bouquet de 

 la Grye, it was not necessary to summon fifty or sixty astronomers to 

 a Congress to arrange for taking mere photographs of stars — a number 

 of jDhotographers provided with instruments like the Henrys could 

 have done all that without a congress. It was very strongly felt that 

 the true raison d'etre of the Conference was to secure astronomical 

 data, precise and exact as the operations of astronomers should be. 



Accordingly they resolved that — 



" In addition to the duplicate series of plates giving all the stars 

 to the 14:th magnitude, there should be a series of plates of shorter 

 exposure to insure a greater accuracy in the micrometric measui'e- 

 ment of the standard stars, and to render the construction of a cata- 

 logue possible. The plates intended for the formation of the cata- 

 logue shall contain all the stars to the 11th magnitude inclusive." 

 That is to say, it was determined to catalogue the absolute places of 

 stars to the 11th magnitude. 



But no photographic plate of itself gives us any information about 

 the absolute places of stars, though it gives the means to determine 

 the relative positions of the stars on the limited area of each plate ; 

 you must trust to the old-fashioned meridian observations to determine 

 the absolute places of the brighter stars on each plate, and then mea- 

 sure the position of the fainter stars relative to these standard stars. 



