1867.] for the Verification of Sextants. 3 



The collimator E is at tlie principal focus of the lens a, and the colli- 

 mator G- at that of the lens h. 



Elevation of Collimators, showing wires viewed from outside the circle. 



Plan of Cooke's Apparatus for Verifying Sextants at the Kew Observatory. 



A. Double collimators. D. Candles for illummating wires. 



B. Table for holding sextants. E. Slate to which collimators are bolted. 



C. Sextant. F & G-. Wires in collimators. 



Furthermore, the lenses are so adjusted that the two lines, the one of 

 which is that proceeding from the centre of the collimator F to the 

 centre of the lens a, and the other that proceeding from the centre of 

 the collimator Gr to the centre of the lens shall be parallel to one 

 another. 



This condition is fulfilled in the following manner : — A telescope 

 having an object-glass suificiently large to embrace at once the two 

 collimators a and h, is focused by means of a star for an object infi- 

 nitely distant. It is then used as an instrument wherewith to view 

 these collimators previously illuminated ; if they appear in focus, it 

 follows that they are to be optically regarded as infinitely distant 

 bodies, and thus that they are accurately at the principal foci of their 

 respective lenses, ^ 



In the next place, things are so adjusted that the vertical collimator 

 shall appear to bisect the cross-wire collimator in the field of view of 



B 2 



