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by reflection will give the true angular diftance be- 

 tween them reduced to the centre of the index- 

 glafs. 



It is to be obferved, that if the mark be fet up 

 at the fame diftance from the quadrant as the direct 

 object is, there will be no occafion to invert the plane 

 of the quadrant, but the obferyer need only make the 

 image of the mark coincide with the direct object, 

 then turn himfelf half round, and now taking the 

 mark for the direct object caufe the image of the 

 former direct object to coincide with the mark, the 

 divided arch of the quadrant being kept upwards, 

 and the place of the centre of the quadrant remain- 

 ing alio the fame in both cafes : half the difference 

 of the fum of the two inftrumental angles from 360 

 degrees will be the correction of the adjuftment 

 as before. 



Should only one of the objects be near, and the 

 other remote (that is to fay, half a mile diftant or 

 more) let the diftant object be taken for the direct 

 one, and the near object for the reflected one j and 

 the true diftance of the objects as feen from the 

 centre of the index-glafs will be obtained without 

 requiring any correction, whether it be the back or 

 fore obfervation that is made ufe of; only obferving, 

 as ufual, to take the fupplement of what is fhewn 

 upon the arch to 180 degrees in the back-obfer- 

 vation. 



XV. Account 



