SANTIAGO OBSERVATIONS. 



xlvii 



For "dark-field" illumination there are two lamps suspended on gimbals, that throw light 

 through the small tubes seen on the plate in advance of the micrometer ; but there is no means 

 for illuminating the field. When unpacked, all the wires put in by the maker were found to 

 have been broken, and the most available material from which to insert a new system was an 

 exceedingly delicate and uniform fibre of unspun Chinese silk. 



From the 6th of December, when the instrument was fully mounted, to the evening of the 

 10th, was passed in becoming familiar with its peculiarities, and bringing it into approximate 

 adjustment, correcting its errors one by one to nearly minimum amounts. It is unnecessary to 

 transcribe the observations made during the several approximations, but only those which were 

 just prior to commencing the appointed series of diiferentials. After the observations of Decem- 

 ber 10th, none of the screws were ever disturbed. 



Observations to determine the position of the Instrument. 



Let d and d' represent the readings of the declination-circle, d when that circle is to the east 

 of the stand; t and t' the times of observation, t when the declination-circle is east; Ti and Ji 

 the corresponding hour-circle readings; ;rthe polar distance of the star observed, ? its refraction 

 in altitude, and g' its refraction in polar distance: then, if we call the index-error of the decli- 

 nation-circle e, the collimation-error of the telescope c, the vertical error of the polar axis x, 

 and its azimuthal error y; 



e = l(d + d> 360) ; 



c=J 

 when the star is in the meridian, 



and when the star is six hours from the meridian, 



it. 



