4 o6 THE POPULAR SCIENCE MONTHLY. 



Pleiades and Praesepe clusters. The measuring machine was 

 improved in 1868 by using a glass scale, one division of which 

 was equal to ten revolutions of the micrometer screw. Mr. 

 Rutherfurd continued his photographic work for twenty years, 

 or till 1877, after which year no photographs were taken by him. 

 " Mr. Rutherfurd," says Prof. Gould, " was the originator and the 

 introducer of the photographic method of observation. To him 

 is due the first idea and employment of an object-glass construct- 

 ed for employing the chemical rays rather than the visual ones ; 

 as also, later, that of the ' photographic corrector ' for adapting 

 an ordinary object-glass to its best use in securing sharp defini- 

 tion of the stars upon the sensitive plate. He personally planned 

 the construction of the first instruments of these classes, pre- 

 scribed the curves for the several surfaces of the lenses, and su- 

 perintended the preparation of the object-glasses, which were made, 

 with the assistance of Mr. Zitz's son, in his own house, by methods 

 devised and made practical by himself alone. So, too, was it he 

 who introduced the precautions by which the sensitive film was 

 guarded against distortion ; it was he who first devised and con- 

 structed micrometric apparatus for measuring the impressions 

 upon the plates ; and he who first put this apparatus into practi- 

 cal use in executing his measurements. The large and delicate 

 micrometer screws were made by him or under his constant su- 

 pervision, at his dwelling-house in this city [New York], and the 

 measurements were effected in his study/' It is related by a 

 writer in Nature, in illustration of the pains he took to secure 

 the utmost perfection in the cutting of the threads of his microm- 

 eter screw, that he took three years to make a single screw. " In 

 order to test the quality of his work, it struck him that it would 

 be a happy thought to see if it would enable him to rule a grating. 

 He accordingly set the apparatus up in his workroom, and by 

 means of an automatic arrangement kept it going all night, as at 

 that time the local vibrations were fewest. The result was that 

 he was able to make the most perfect gratings [then] known." 



For many years Prof. Gould says Mr. Rutherfurd labored at 

 the photographic method of observation without the sympathy or 

 encouraging faith of astronomers generally ; " and in 1865 he did 

 me the honor of placing in my hands a large number of measure- 

 ments, and giving me permission to study and compute them. 

 They had been made in his house, with apparatus designed and 

 in great degree constructed by himself, from photographs which 

 he had personally taken by aid of the telescope which he had 

 himself devised and which was also in his house." At the session 

 of the National Academy of Sciences, held in Northampton, 

 Mass., in August, 1866, Prof. Gould presented a memoir contain- 

 ing the results of computations, made from these data, for deter- 



