OBSERVATIONS WITH THE ZENITH TELESCOPE. 



299 



reobserved (excepting three pairs) in connection with the new one, for the purpose of 

 connecting the two series, thus insuring continuity in the results. 



Third. — Series extending from October 10, 1892, to December 27, 1893. The 

 same list, excepting a few objectionable pairs which were replaced when possible by 

 more favorable ones, was employed as above — 107 pairs in all. 



Fourth. — Observation upon 41 pairs, forming four groups, employing the method of 

 Kiistner.* All of these pairs are found in the list used in Series Three. These observa- 

 tions extended from January 19, 1894, to August 19, 1895. As the results of this 

 series have already been published, f no further comment is called for in this connection. 



FIEST SERIES. 



The Instrument. 



A Zenith Telescope of three inches aperture and forty-one inches focal length, made 

 by E. & Gr. W. Blunt, of New York, was employed throughout the entire campaign. It 

 is said to have been purchased at second-hand from the U. S. Coast Survey in 1868, or 

 thereabout. Upon taking charge of the Department of Astronomy at Lehigh University 

 in 1875, I found the instrument in a dilapidated condition. Before attempting any 

 serious work it was sent to Edw. Kahler of Washington, who furnished a new level and 

 made some other repairs. 



Level Valve 



This was investigated by attaching the tube to the Mural Circle of the U. S. Naval 

 Observatory. Though the mean value may be found in this way with reasonable 

 accuracy, the method is not adapted to investigating the question of the uniformity of 

 curvature of the tube. Four of the six micrometers with which the circle was provided 

 were read for each position of the bubble. 



The mean of fifteen such determinations gave for one division of the level the value 



1".06 



No further attention was given to the matter until 1888, when a level-trier gave 

 the value 



0".80 



* Attronomisohe IVaehrichten, Bd. L26, No. 8015. 



•|- Transactions of the Ambkican Philosophical Society, Vol. xx, Philadelphia, 1901. 



