258 TRANSACTIONS OF THE [mAY 17^ 



and distributed at his own expense about twenty, and more are 

 soon to follow. 



But the old-fashioned way of cataloguing and charting the 

 stars is obviously inadequate to the present needs of astronomy, 

 and a new era has begun. While, hereafter, as hitherto, the 

 principal stars, several iiundred of them, will be observed even 

 more assiduously and carefully than ever before, with the me- 

 ridian circle or similar instruments, the })hotographic jilate will 

 supersede the eye for all the rest. It is now easily possible to 

 pliotograph stars down to tlie thirteenth or fourteenth magni- 

 tude, and to cover a space of 2^° square on a single plate. The 

 remarkable thirteen and one-half inch instrument constructed by 

 the Henry Brothers, for the Paris observatory, and first brought 

 into use last August, does this perfectly. Instruments very 

 similar, but smaller, lately set up at Harvard College, at the 

 Caj)e of Good Hope, and at Liverpool, while they do not reach 

 so faint stars, cover more ground at a time. 



Negotiations are already under way to secure the co-operation 

 of a number of observatories for a photographic survey of the 

 heavens; and it is probable that, after somepreliminary consulta- 

 tion and before very long, it will be actually in progress. According 

 to Struve's estimates, it could be accomplished in about ten or 

 twelve years, even on the Paris scale, by the combined efforts of 

 fourteen or fifteen establishments. Orders have already been given 

 to the Henry Brothers, by Dom Pedro, of Brazil, and Mr. Com- 

 mon, of England, for instruments precisely like the one at Paris. 

 Americans, and New Yorkers especially, may well take a i)eculiar 

 interest in Astronomical photography, since it was at Cambridge,, 

 in 1861, that the first star-photographs were ever made, and 

 here in New York, Rutherfurd and Draper were among the earliest 

 and. most successful workers: in the observatory above us is now 

 mounted the very instrument with which lUitherfurd made his 

 unrivalled pictures of the moon and his plates of the Pleiades, 

 more than twenty years ago. 



During the past ten years, stellar photometry has become al- 

 most a new science. Its foundations, indeed, were laid by J. 

 Herschel, Seidel, Wolff, and ZuUner, before 1870, and the mag- 

 nitudes of some two hundred stars had been measured, and the 

 law of atmospheric absor})tion determined. But the great work 

 of Pickering, at Harvard, in the invention and perfecting of 

 new instruments, and his Harvard photometry, which gives us 

 a careful measurement of the brightness of all the naked-eye 

 stars of the northern hemisphere, marks an e})och. And he is 

 pushing on, and has already well under way the measurement of 

 the 300,000 stars of Argelander's Durchmusterung. Nor must 

 we omit to mention Pritchard, of England, whose name has just. 



