SCIENCE 



NEW YORK, JULY 17, 1891. 



THE PHOTOCHRONOGRAPH. 



The observations described and discussed in a recent publication 

 of the Georgetown College observatory bad their oriyin in a visit 

 made by Professor Frank H. Biselovp and Mr. G. Saegmuller to 

 that observatory in the summer of 1889. Their object was to use 

 the observatory's five-inch equatorial for a series of experiments 

 in pliotographing star transits. All the resources of the observa- 

 tory were most cordially placed at their disposal by the director, 

 the Rev J. G. Hagen, and on Aug. 13, 1889, the first apparatus 

 was tested. 



The experiments were repeated on Aug. 26 with an improved 

 apparatus, and were then broken off, for the time being, by the 

 appointment of Professor Bigelow to the West African eclipse ex- 

 pedition. 



The purpose of the method of observation described is the re- 

 moval of the " personal equation " in transit observations, by 

 means of photography. One night. Professor Bigelow, to whom 

 the idea is due, and Mr. Saegmiiller, an instrument-maker of 

 Washington, were sitting with the director of the observatory at 

 the table in the library, and consulting as to the best way of put- 

 ting the idea to a test. The long focus of the equatorial and the 

 electrical connections for the time-signals and incandescent lamps 

 came in very handy for the purpose. The tirst camera was soon 

 constructed and screwed to the eye end of the telescope, and a few 

 evenings later the siar Alpha Aquilae vvas made to trace its diur- 

 nal motion on a small plate not quite two inches square, while the 

 sidenal clock made the whole camera move in a vertical direction 

 once every second. Finally, the spider-lines of the micrometer 

 were photographed on the same plate by means of an incandes- 

 cent lamp held for a few seconds before the object-glass. The 

 development of the first plate, in the dark-room of the cellar, was 

 watched with great expectation, and, to the satisfaction of all the 

 bystanders, brought forth two parallel trails, broken into dashes, 

 each representmg a second of time, and the whole reticule of the 

 micrometer lines. The first apparatus was soon superseded by a 

 second, and the second by a third, each being improved as the 

 experiments suggested. 



Shortly afterward Professor George A. Fargis was attached to 

 the observatory, and at once put in charge of the Ertel transit- 

 instrument. The " photochronograph " in its present shape, and 

 and the improved method of photographing the reticule wires 

 without injury to the star-trails, are entirely his inventions. 



The principal idea in photographing the transits of stars is to 

 translate the phenomenon of an object moving over tbe field of 

 view in a telescope — the instant at which it passes a thread being 

 noted by eye or ear, or by the chronograph — into a picture of the 

 salne which can be measured leisurely as a fixed object. The 

 error of the personal equation depends ultimately upon the diflEer- 

 ence of time required by the eye to receive the impression of a 

 bright object and to let the same go. Impression is more rapid 

 than the letting go, and tbe relative times of retention vary with 

 individuals, and with the variable physical condition of the indi- 

 viduals. As stars are momentarily occulted behind a thread in 

 transit, this element of retention throws the apparent place of the 

 star beyond the thread ; hence eye and ear observations, being 

 taken at intervals from the thread, Tith the star in full vision, are 

 more accurate, and need a positive correction usually to be re- 

 duced to transits by chronograph taken on tbe thread. 



A photographic plate reduces to a minimum this correction, 

 both as to its amount and its variability ; and whatever other cor- 

 rections may be introduced by the process, they are mechanical 

 and can be obtained by the discussion of results. 



For the work of makinc; a standard catalogue of bright stars 

 for fundamental differential comparisons, for longitude determi- 

 nations, for latitude, and also for physical observations in labora- 

 tories it seems very important to eliminate by some process the 

 error of the personal equation, which is in fact the greatest source 

 of error in all such measures. 



The first experiments to determine time by photographs of star 

 transits were made by Professor E. C. Pickering, at the Harvard 

 College observatory, in January, 1886 the account of the work 

 being given in the Memoirs of the American Academy (Vol. XL,, 

 p. 318). 



The star images of the Pleiades were allowed to trail over a sen- 

 sitive plate, the eight inch Bache telescope being used, for known 

 intervals of time. It was fotmd that microscoi:)e settings on the 

 marks could be made with a probable error of only 0'.03, thus 

 indicating the possibilities opened up by the method. 



It now remained to apply it to transits in some way available in 

 accurate reductions. There are two steps to be taken: (1) that 

 which should connect the star trail definitely with the astronomi- 

 cal clock; (2) another enabling the transit to be referred to the 

 collimation axis of the telescope. Both must be simple, and free 

 from all important objections. At the suggestion of Professor 

 Pickering and Mr. Willard P. Gerrish, it was. in the summer of 

 1888, at the Harvard College observatory, attempted to solve the 

 first in the following way. A small plate was attached to the 

 armature of a magnet by which a movement up and down, per- 

 pendicular to the star trail through a very small interval, could 

 be communicatad to it by making and breaking the circuit at fixed 

 intervals, either by hand or by the clock, the latter requiring a 

 commutator in which the makes and the breaks should be of equal 

 lengths. The effect was to leave on the plate a pair of dotted 

 lines close together the dots alternating in the perpendicular di- 

 rection The beginnings of the intervals and the endings were 

 definitely marked, and settings of a micrometer thread could be 

 made on them very accurately, the probable error being not greater 

 than 0'.03 in second-intervals. 



Professor Bigelow constructed an apparatus for testing this 

 process, the plate being kept in a paper-holder inserted in a slide 

 attached to tbe rocking armature which responded to tbe currents 

 in the magnet. The telescope used was the eight-inch equatorial 

 and the current was made and broken by hand. In the autumn 

 Mr. Gerrish constructed a commutator by which the clock made 

 the motions of the plate automatically. 



Omitting for the moment mention of the second part of the 

 process, for the sake of the chronological order, the next piece of" 

 apparatus was constructed by Mr. Saeamiiller of Washington,. 

 D.C., at the suggestion of Pi'ofessor Bigelow, and was tried a few 

 nights at the Georgetown observatory in the summer of 1889, by 

 the kindness of the director, the Rev. J. G. Hagen. This piece- 

 embodied an important improvement, while retaining the alter- 

 nating motion of the plate. A frame was made to can-y a small: 

 plate-holder, which could be very readily interchanged with a 

 common observing eye-piece. The star having been received into- 

 the field and adjusted by the direct vision, in five seconds the 

 plate could be placed to receive the transit, this being a great 

 practical improvement, as otherwise the adjustment of the stat 

 trail depended upon the finder to tbe telescope. 



The eclipse expedition to West \frica, 1889, withdrew Professor 

 Bigelow's attention from the subject, and further development 

 devolved wholly upon the Georgetown College observatory. 



Returning to the second divi-iion of the experiment, namely, the 

 referring this trail to the middle wire and thence to the collima- 

 tion axis of the telescope, without which the observation would 

 be worthless, a number of combmatioos were discussed at the 

 Harvard College observatory. Professor Bigelow's first expert- 



