502 



Proceedings of the Royal Irish Academy. 



before the eye-piece, lie made the star go horizontally in the same 

 direction dnrino; 20 transits. The results are as follows : — 



1864. 



From Right 



From Left 





to Left . 



to Right. 



May 11 



4 0^-08 



4 0^-17 



12 



+ 0-11 



4 0-12 



1-2 



+ 0-09 



4 0-16 



12 



+ -13 



4 0-15 



June 2 



+ 0-12 



4 0-18 



8 



+ -11 



4 0-12 



8 



4 0-10 



4 0-15 



8 



4 -10 



4 0-14 



July 16 



4 0-09 



4 0-13 



23 



4 -OS 



4 0-13 



23 



4 -09 



4 0-13 



Mean 



4 0-10 



4 0-14 



There is here a distinct though slight difference, and the mean 

 result must be considered reliable, as the arrangement of the observa- 

 tions excluded the influence of faults in the apparatus.* Similar very 

 small differences were found with Hirsch's apparatus (using the 

 chronographic method). 



Plantamour 4 0^-01 ± 0^-02 



Rudolph Wolf 4 -04 + -01 

 Hirsch 4 -06 ± -03 



E,etrog. motion 

 4 direct motion. 



It is remarkable that these small differences have the same sign as C. 

 "Wolf's.f 



In Leyden there has been made a series of experiments with the 

 second time-coUimator, but only one of the observers found a slight differ- 

 ence which was very uncertain, as the chronographic method had only 

 lately been introduced at the observatory.;]: Researches on this sub- 

 ject have also been made by Wagner, in Pulkowa, with a time colli- 

 mator on Kaiser's principle, but his observations, which he has been 

 kind enough to communicate to me, and of which we shall hear more 

 further on, show no perceptible influence of the direction of the star's 

 motion. We have heard of no other investigations of this kind except 

 of a series of transits ot artificial stars in perpendicular direction (ana- 

 logous to transits of zenith-stars in a broken telescope), taken in 

 Leyden, but the four observers, all of whom had a very small absolute 

 error, found it to be a matter of no consequence whether the star was 

 going upwards or downwards. § Several observations of artificial stars, 



* Annales de 1' Observatoire de Paris (Meinoires) viii., p. 174. 



t DeterminationTelegraphiquede laDifference de Longitude entre E,igM-Kiilm, 

 Ziirich et Neuchalel, Geneve et Bale, 1871, p. 187. 



X Verslagon en MededeUngen der K. Acadcmie von "Wetenschappen, 2 ser., ii., 

 p. 235. 



§ Ibid. 



