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by refledtion 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 direft: 
objedt is, there will be no occafton to invert the plane 
of the quadrant, but the obferver need only make the 
image of the mark coincide with the diredt objedt, 
then turn himfelf half round, and now taking the 
mark for the diredt objedt caufe the image of the 
former diredt objedl 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 alfo the fame in both cafes : half the difference 
of the fum of the two inftrumental angles from 360 
degrees will be the corredtion of the adjuftment 
as before. 
Should only one of the objedts be near, and the 
other remote (that is to fay, half a mile diftant or 
more) let the diftant objedt be taken for the diredt 
one, and the near objedt for the refledted one ; and 
the true diftance of the objedts as feen from the 
centre of the index-glafs will be obtained without 
requiring any corredtion, 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 
