1887.] on the Applications of Photography in Astronomy. 163 



Every detail of their process of working has been evolved by 

 themselves ; they employ no assistant, and their every appliance is 

 simple and practical in a degree which I can only compare with the 

 simple and practical character of the men who designed it. 



Such were the men above all others to develop the application of 

 photography to the charting of the heavens. They had high appre- 

 ciation of the value of the work which they were about to undertake, 

 they had the fullest knowledge of the requirements of the case, and 

 they had the practical skill which enabled them to perfect the neces- 

 sary apparatus. Their first attempts were made with a telescope of 

 six inches aperture (the object-glass being specially ground for 

 photographic work), and the tube was temporarily adapted to an 

 existing equatorial stand. 



With an exposure of forty-five minutes, pictures of stars were 

 obtained to the 12th magnitude, in which the star discs were quite 

 round and sharply defined. 



Fully appreciating the beauty of this result, and seeing its 

 importance. Admiral Mouchez boldly faced many administrative 

 difficulties, and accepted without delay the proposals of the brothers 

 Henry to construct an object-glass of thirteen inches aperture and 

 about eleven feet focal length, as well as the offer of M. Gautier to 

 mount the same on a suitable stand. The new instrument was 

 mounted in May 1885. A photograph of the complete instrument is 

 now on the screen. 



Both from an optical as well as a ^mechanical point of view, the 

 new instrument was admirably adapted for its intended work, and 

 the results obtained by the brothers Henry, and rapidly published 

 and circulated by Admiral Mouchez, at once astonished and de- 

 lighted the astronomical world. 



I now show a few of the more remarkable of these star pictures 

 on the screen. 



After such results as these there was no longer room for doubt 

 or delay. The exquisite precision of these pictures, the sharpness 

 and roundness of the images of the stars, and the results of actual 

 measurement on the plates, proved that all necessary accuracy had 

 been attained. 



The means of rapidly obtaining the data for an accurate survey 

 of the heavens on a very large scale were now within the reach of 

 astronomers, and the time for decisive action had arrived. 



The work, however, was too extensive to be undertaken at a 

 single observatory, or even by a single country, and it was agreed 

 on all hands that international co-operation was essential for its 

 execution in a sufficiently short space of time. 



I need not enter into the details of preliminary consultation or 

 correspondence, but at last a time was fixed, and invitations were 

 issued by Admiral Mouchez, Director of the Paris Observatory, under 

 the auspices of the Paris Academy of Sciences, for an International 

 Congress of Astronomers to be held at Paris. 



M 2 



