47] THE OBSERVATORY OF CAMBRIDGE. 379 



at the middle vertical wire. For the Polar stars the concluded reading- 

 applies to the time of meridian passage. 



The Circle reading corresponding to the position of the Telescope when 

 directed exactly to the zenith is called the Zenith Point. 



The adopted Zenith point is obtained by means of the collimating 

 eye-piece, and is therefore more strictly the Circle reading corresponding 

 to the Nadir point increased by 180. 



The Collimating eye-piece employed is of the same form as that used 

 by Professor Challis, and consists of a common inverting microscope of 

 three lenses, to which is attached, beyond the third lens, a piece of plate- 

 glass, inclined at an angle of 45 to the axis of the microscope. The 

 eye-piece of the Telescope being removed, this apparatus is put in its 

 place, so that the plate-glass is between the wires and the microscope ; 

 and when the Telescope is directed vertically to a trough of mercury, 

 the wires and their images by reflection become visible as dark lines on 

 a bright ground, by throwing the light of a lamp on the plate-glass. 



The Micrometer reading for coincidence of the micrometer-wire with 

 its image is deduced from at least six readings for coincidence, or for 

 alternate contact. 



The Microscope readings for the determination of the Zenith point 

 are inserted among those for the observations of the celestial objects 

 named in the second column. The concluded Circle reading obtained by 

 reducing an observation of Nadir point in the same manner as the other 

 observations are reduced, and then increasing the result by 180, is in 

 general the adopted Zenith point. The limits within which any value is 

 used are indicated by bars across the column of " concluded circle readings." 

 If two observations of Zenith point occur within the same limits, the 

 value used is the mean between the two results. 



The temperature of the Circle room at the times of taking the 

 Zenith point is given in the Table of observations of Runs. 



The apparent Zenith distance in the direct observation of any object 

 is the algebraic excess of the concluded Circle reading above the adopted 

 Zenith point, and for a reflection observation it is the algebraic excess 

 of the Nadir point above the concluded Circle reading. The object is 

 South or North of the zenith according as the excess is in either case 



482 



