Dreyer — On Astronomical Transit Ohscrvations. 



507 



1 . "WTien the field of tlie telescope is illuminated from the west, 

 upper transits are observed, too early if the eye-piece is drawn out too 

 far for the observer's sight, and too late if it is pushed in too much. 



2. With eastern illumination the case is reversed ; the transit is 

 taken too late with the eye-piece drawn out, and too early with it 

 pushed in. 



3. If the field is illuminated by the diffused daylight, the position 

 of the eye-piece has no influence on the personal error ; and this error 

 is also, on the other hand, independent of the direction of the illumina- 

 tion, if the eye-piece is exactly adjusted. 



As during the above-quoted observations by R. "Wolf and Hirsch, 

 the eye-piece had always been drawn out too far for the sight of the 

 former, it was now clear why he had observed too late in Neuf chatel, 

 where the illumination came from the east, and too early in Zurich, 

 where it came from the west.* ^Notwithstanding all this, the problem 

 was not yet solved, as this effect of the position of the eye-piece and of 

 the illumination was unexplained, and besides, how was it that Hirsch 

 and the long-sighted Plantamour, in their numerous determinations of 

 their personal equations, had never remarked such an effect ? These 

 questions were important enough to deserve a nearer investigation, 

 and such a one was, therefore, made by Hirsch and his assistant, 

 Schmidt, in l^Teufchatel, during the spring of 1870, as followsf : — 

 Each observer observed a certain number of stars over all the wires, 

 having the eye-piece adjusted for his eye during the transit across half 

 the wires, and having it drawn out or pushed in a little during the 

 transit over the remaining half. The eye-piece was in either case 

 moved to an equal distance from the normal position, and the part of 

 the wires observed with the adjusted eye-piece was constantly 

 exchanged for another. The observations, which are communicated in 

 the " Astron. Mittheilungen," xxvi., gave the following results. By 

 a we designate adjusted eye-piece, by d drawn out, and by p pushed in. 

 The illumination came from the east : — 



1870. 



Number of 

 Stars. 



Difference between 

 the Transits, 

 reduced to the 

 Central Wire. 



Mean Error. 



Observer. 



April 20, 

 April 22, 

 April 25, 

 April 26, 



14 

 10 

 U 

 15 



a-d = -V 0^-259 

 a-d = + Q -280 

 «-j9 = _0 -276 

 ^;_(^ = + •464 



+ 0»-021 

 -019 

 -009 

 -Oil 



S. 

 H. 



S. 

 S. 



* In both the instruments in use the inumination was from east or west sent 

 down to the wires by a small min'or under an angle of 45^ with the axis. 



t Determination telegraphique de la difference de longitude entre la station 

 astronomique de Eighi-Kulm et les Observ'atoires de Ziirich et de Neuf chatel. Geneve, 

 1871, pp. 171 and ±'oU. 



SCIEXCE. 3 F 



R. I. A. PROC, SER. ir., VOL. II. 



