VOL. 45 Parts III and IV 



Smithsonian 

 Miscellaneous Collections 



Quarterly Issue October-December, 1903 



A METHOD OF AVOIDING PERSONAL EQUATION IN 

 TRANSIT OBSERVATIONS 



By S. p. LANGLEY 



Many years ago the writer devised and published^ preliminary 

 accounts of a method for avoiding the so-called " personal equa- 

 tion " in observations of the time of transit of stars. The following 

 quotation from the article cited will give the fundamental idea 

 described in the former article, to which the present one adds some 

 more recent experiments and slight modifications. The writer, 

 after speaking of the comparative merits of and objections to a 

 photographic method, and one where the star is seen intermittently 

 projected on wires which are illuminated by consecutive flashes, 

 observes : 



" There is one particular case, however, where the result is the 

 same for both, that, namely, where when the flash comes, the star 

 is on the wire and bisected by it ; in this case we know its position 

 as accurately by the eye, as if we bisected its image on the plate 

 by the wire of our micrometer. If we suppose, then, that by a 

 happy accident, the flash came just as the star was crossing the 

 first wire, this wire would be sharply defined on the disc of the 

 star and bisecting it, and a simultaneous record on the chronograph 

 (made without the intervention of the observer) would evidently 

 give us the same result as though the star had recorded its own 

 passage by an electrical contact. Further, we may particularly 

 notice that it is immaterial whether the star was at rest, when thus 

 seen, or in motion. Now what we have just supposed as a single 

 case of a favorable chance among hundreds, it will be our task to 

 make occur, whatever the wire interval, and for any star observed." 



In illustration let it be imagined that the observer is watching 

 the passage of the star through the field of his telescope, but that 



^American Journal of Science and Arts, July, 1877, pages 55-60. 



22 c 



