May 15, 1896.] 



SCIENCE. 



719 



The work of Lewis M. Eutherfurd, of this 

 city, an honored member of this Academy 

 for many years, in photographing stellar 

 clusters and in measuring the plates with a 

 machine of his own devising, was the first 

 serious attempt to use photography in get- 

 ting the exact relations of stars to each 

 other. The recent publications of the Co- 

 lumbia College Observatory show unmis- 

 takably that from these measures relative 

 positions of the highest precision are ob- 

 tainable. The invention of dry plate pho- 

 tography has made the photographic work 

 of the astronomer of to-day much more 

 expeditious, and enables him to secure 

 many more stars on his plates with a given 

 time of exposure. The last years of this 

 century witness the carrying out of a gi- 

 gantic plan for making an enormous cata- 

 logue of the highest precision by the aid of 

 photography and supplementing this cata- 

 logue by a series of charts. 



On April 16, 1887, there met in Paris 56 

 delegates of 17 different countries, to dis- 

 cuss ways and means of carrying out this 

 grand photographic work. The final de- 

 cision was to construct a photographic 

 chart of the heavens of all the stars down 

 to the 14th magnitude. On these plates 

 will appear, it is estimated, some 20,000,000 

 stars. Methods are now being devised to 

 reproduce accurately these chart plates. 

 It was decided also to supplement these 

 chart plates, made with an exposure of 40 

 minutes, by plates of shorter exposures, 

 from measurements of which, with ma- 

 chines of the highest precision, a catalogue 

 is to be prepared. These catalogue plates 

 show the stars down to the 11th magnitude, 

 and the number of stars may reach two 

 millions. Twenty-two thousand plates (du- 

 plicated and overlapping) will be necessary 

 for the catalogue. The work has been 

 going on for several years at 18 observa- 

 tories throughoiit the world (except in the 

 United States) and the photographic part 



will soon be finished. The measurements, 

 of the plates and the calculations based 

 thereon are also being carried on at Paris,. 

 Potsdam, Greenwich and elsewhere. Judg- 

 ing from the work already done, we may 

 confidently expect that nearly all the re- 

 sults will be ready for printing in about ten 

 years. The astronomers of the 20th cen- 

 tury will then be in possession of material 

 which will aid them in studying the prob- 

 lems connected with the construction of the 

 universe of stars. 



The number of stars around us increases 

 with every augmentation in telescopic 

 power, and in time of exposure of photo- 

 graphic plates. The largest telescopes will 

 show perhaps more than 60,000,000 stars. 

 The long exposure photographs (say 12 

 hours) would show many millions more. 

 M. I'Hermite has ' computed the population 

 of the stellar universe from his valuation 

 of stellar light power and finds it, on the 

 assumption that the scattering of the stars 

 is everywhere just as it is in our own 

 neighborhood, to be sixty-six thousand mil- 

 lions! ' This result is ingenious and inter- 

 esting, but depends for its value on the 

 above assumption. 



The task of sidereal astronomy — a stu- 

 pendous one— is this then: ' to investigate 

 the nature, origin and relationships of mil- 

 lions of stars; to inquire into their move- 

 ments among themselves and that of our 

 sun among them,' and ' to assign to each 

 its place and rank in the universal order.' 



Among the great number of interesting 

 problems in sidereal astronomy, let me select 

 two or three of the most important. The 

 catalogues of the highest precision enable 

 the astronomer to determine the positions 

 of various stars at widely different dates. 

 This requires that the catalogues used 

 should be made up from observations at 

 those dates. Now comparing the positions 

 of a star at any two dates a difference will 

 be found depending in amount on the lapse 



