510 



Proceedings of the Royal Irish Academy, 



mour and E. "Wolf. The latter recapitulates the influence of the- 

 reflector in the following way :* — 



OBJECT 



-QLASS 



% 



/#' 



WEST ^\ 



J'''' EAST 



Af 



V 







y / / 



V\''n 



.•-/;-•'•' 



'■>\-- 



6 '' 



* EYE- 



\ 2 



PIECE 3 



Place of 

 the Lamp. 



Position 



of the 



Reflector. 



Field. 



Relative position of the image ilium, by the reflector, 

 with respect to the image on the bright ground. 



E3'e-piece drawn out. Eye-piece pushed in. 



West. 

 East. 



1 

 2 

 3 

 4 

 5 

 6 



Bright. 

 Dark.f 

 Bright. 

 Bright. 

 Dark.f 

 Bright. 



To the right. 



To the left. 

 To the right. 



To the left. 



To the left. 



To the right. 

 To the left. 



To the right. 



"We have here so fully described the results of the investigations 

 -of the Swiss astronomers, because it is evident that they are of very 

 great importance, not only for our knowledge of the personal error, 

 but for practical astronomy in general. They explain most probably 

 the sudden changes in the equation of two observers, which have 

 often been remarked, and they have, besides, confirmed the suggestion 

 that the illumination of the wires causes the difference between the 

 errors in the two positions of a broken telescope, which sometimes 

 appear. Two of the weakest points in the art of observation have 

 -in this way been made clear. And it is easy to avoid the causes 

 of the disturbances, now that they are known. If the observer has 

 ronly adjusted the eye-piece according to his sight, he may be sure 



* Vierteljahrsschi-ift der Naturforschenden Gesellschaft in Ziirich, xv., 1870, 

 p. 249. 



f Only so in the Ziirich instniment, o-wing to the large aperture of the re- 

 flector. 



