THE POSSIBILITIES OF COOPERATION 397 



celestial photography at the Cape of Good Hope. No special 

 photographic telescope was available, but Sir David enlisted 

 the aid of a local photographer, whose camera, strapped to an 

 equatorial telescope, immediately yielded pictures of exceptional 

 value. But even more striking than the image of the comet 

 itself was the dense background of stars simultaneously regis- 

 tered upon these plates. Stellar photographs had been taken 

 before, but they had shown only a few of the brighter stars, 

 and no such demonstration of the boundless possibilities of 

 astronomical photography had ever been encountered. Always 

 alive to new opportunities and keen in the appreciation of new 

 methods, Sir David adopted similar means for the mapping of 

 more than 450,000 stars, whose positions were determined 

 through the cooperation of Professor Kapteyn of Groningen, 

 who measured their images on the photographs. 



Stimulated by this success, the Henry Brothers soon adopted 

 photographic methods for star charting at the Paris Observa- 

 tory, and in 1887 an International Congress, called at Sir 

 David's suggestion, met in Paris to arrange for a general survey 

 of the entire heavens by photography. Fifty-six delegates of 

 seventeen different nationalities resolved to construct a photo- 

 graphic chart of the whole sky, comprising stars down to the 

 fourteenth magnitude, estimated to be twenty millions in num- 

 ber. A standard form of photographic telescope was adopted 

 for use at eighteen observatories scattered over the globe, with 

 results which have appeared in many volumes. These contain 

 the measured positions of the stars, and are supplemented by 

 heliogravure enlargements from the plates, estimated, when 

 complete for the entire atlas of the sky, to form a pile thirty 

 feet high and two tons in weight. 



The great cooperative undertaking just described is one that 

 involves dealing with a task that is too large for a single institu- 

 tion, and therefore calls for a division of labor among a number 

 of participants. It should be remembered, however, that a very 

 different mode of attacking such a problem may be employed. 

 In fact, although the difference between the two methods may 



