Dreyer — On Astrono7nicaI Transit Observatio)is. 487 



Before entering on the examination of the different results ■which 

 can be extracted fi'om modern researches, Tve shall shortly consider the 

 methods by which the personal difference between two observers, and 

 the absolute personal error of a single observer, may be found. 



The most convenient, as well as the simplest way, to find the 

 equation between two persons is to let the one observe the transits of 

 stars over the one half of the wii'es in the telescope, and the other person 

 observe the transits over the remaining wires. The single transits, 

 reduced to the middle wire, give immediately the equation. By 

 changiag the half of the system of wires observed by each person, the 

 influence of faults in the distances of the wires is eliminated. A 

 change of this method is the use of a binociilar eye-piece, which, by a 

 prism, divides the rays coming from the object-glass into two parts, 

 so that two observers at the same time may observe the transit of a 

 star across all the wires. This method has for some time been used in 

 Greenwich, but it causes often a change in the personal error to arise 

 from the position of the observer, east or west ; therefore, it cannot be 

 recommended.'-' It is also in another way possible to use all the wires, 

 by letting the two persons observe the projected image of the sun on 

 a piece of white paper, f But as the observation of the luminous edge 

 of the sun is very different from that of a star, a personal difference 

 in the former need not be identical with that in the latter, so that a 

 control by star-observations, at all events, is necessary. 



Besides these methods — of which the fijL'st one is the simplest and 

 the one most commonly used — several other methods of finding per- 

 sonal differences may be used. "When Bessel, for instance, compared 

 himself with AYalbeck, each of them observed five stars a day, and 

 every second day the same. By comparing the observations made on 

 two consecutive days, two values of the clock-rate were obtained, the 

 difference of which was equal to the double personal equation. J The 

 equation B. -Argelander was, at the same time, found in another 

 manner. Bessel had, in 1821, six times observed seven stars (used 

 by Bradley and Maskelyne for determining the coUimation error of 

 the Greenwich quadi'ant) ; Ai'gelander observed twice the same stars 

 in 1823, while Bessel found the clock-error. A. found now the right 

 ascensions to be larger than B. had done : the equation B. - A. was, 

 therefore, on an average = - P"22. A similar method is used in 

 Greenwich, where the different observers at the transit insti-ument, 

 from a series of stars, determine the clock-error separately, and reduce 



* "We shall afterwards come back to this peculiar case. 



t About this method see "Washington Observations, i. (for 1845), p. 49 ; 

 Monthly Not., R. A. S., six., p. 338; Monatsberichte der Berliner Academie^ 

 1858, p. 615. "We shall also later come back to the solar observations. 



J Konigsberger Beob., viii., p. 4. B. compared himself in 1832 with Busch 

 and Argelander in the same way. — Ibid.,xviii., p. 1. 



