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 constructed 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 definition of the stars upon the sensitive plate. He personally planned the construction of the first instruments of these classes, prescribed the curves for the several surfaces of the lenses, and superintended 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 constructed micrometric apparatus for measuring the impressions upon the plates; and he who first put this apparatus into practical use in executing his measurements. The large and delicate micrometer screws were made by him or under his constant supervision, 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 micrometer 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 measurements, 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 containing the results of computations, made from these data, for deter-
