508 Proceedings of the Royal Irish Academy. 



These rmmbers allow of no doubt as to the reality of the influence 

 of the eye-piece : besides, the extent of this influence must be inde- 

 pendent of the observer (as both H. and S. found nearly the same 

 result), and it seems to be proportional with the distance of the eye- 

 piece from its adjustment. The result found in this manner in IS'euf- 

 chatel by illumination from the eastern end of the axis was identically 

 the same as the one found in Zurich by illumination from the west, the 

 transit having been taken too late, while the eye-piece was too near 

 the wires, and too early, when it was too far from them. 



The results were accordingly in perfect opposition to the observa- 

 tions taken in Ziirich, and it seemed, therefore, that the placing of the 

 lamp at one or the other end of the axis could not be the cause of the 

 change in the anomaly. As the matter, therefore, seemed worth a closer 

 examination, Hirsch took a series of observations with his artificial 

 apparatus.*' From these it was again seen that the day observations 

 are independent of the position of the eye-piece, while the night obser- 

 vations showed not only again the influence of the eye-piece, but also 

 that the variations are opposite when the lamp is placed east and west. 

 And this time the influence of the illumination, strange enough, was the 

 same as that in Zurich, and opposed to the one that only a few days 

 previously had been found in Neuf chatel. Por the transits were now 

 observed with illumination from the east. 



By H. too early (on an average 0^*1 8) witli eye-piece pushed in. 

 )j '^^ >) >> (. ?) )) )> ^ 'iyj ,, ,, ,, ,, ,, 

 ,, H. ,, late ( ,, ,, ,, Os-12) ,, ,, ,, ch'awnout. 



)! *^- >) ;> \ Jj j> )J ^ 'lOj ,, ,f ,, ,, ,, 



But observed with illumination from the west, 



By H. too late (on an average 0^-21) with eye-piece pushed in. 

 „' S. ,, early („ „ ,, 0-24) „ „ „ di'awnout. 



The question as to koto the position of the eye-piece influences the 

 observation seemed, therefore, to become more and more abstruse, while 

 the existence of this influence was constantly felt if the field was 

 illuminated by artificial light. "What remained was to explain the 

 contradiction between the Xeufchatel observations of natural and arti- 

 ficial stars, with respect to the direction of the influence (retarding or 

 accelerating) for the same position of the lamp and the eye-piece. 



An observation by Hirsch gave at last the key to all the phe- 

 nomena. The apparatus for the artificial star was in ISTeufchatel 

 placed before the meridian-mark, and when the telescope was directed 

 towards the latter, the middle of the field was seen strongly illumi- 

 nated by the gas-flame behind it, so that the wire in the telescope, 

 across which the transits were observed, was projected partly on this 



* Communicated in his " Difference de Longitude, Eighi-Kulm," &c. 



