[ 120 ] 
already well adjufted by a diftant objeCt, and you do 
not chufe to alter it by adjusting it by a near one, 
move the index, and bring the image of the near 
direcft objeCt to coincide with the fame feen direCtly, 
and the number of minutes by which (o) of the 
index Hands to the right hand of (o) of the quadrant 
upon the arch of the excefs is the correction, which 
added to the angle meafured by the arch of the qua- 
drant between this direCt objeCt and any other objeCt 
feen by reflection will give the true angular diftance 
between them reduced to the centre of the index- 
glafs. 
To find the angular difiance between two near objeCts 
by the back-obfervation. 
It is fuppofed that the-horizon-glafs is truly 
adjufted ; if it is not, let it be fo. Obferve the 
diftance of the objeCts by the back-obfervation, and 
take the fupplement of the degrees and minutes 
fianding upon the arch to 1 80 degrees, which call 
the inftrumental angular difiance of the objeCts ; this 
is to be corrected as follows. Keep the centre of the 
quadrant or index-glafs in the fame place as it had in 
the foregoing obfervation, and obferve the diftance 
between the near objeCt, which has been juft taken 
as the direCt objeCt, and fome diftant objeCt, twice; 
by making both objeCts to be the direCt and reflected 
ones alternately, holding the divided arch upwards in 
one cafe and downwards in the other, ftill preferving 
the place of the centre of the quadrant. The 
difference of thefe two obfervations will be the cor- 
rection, _which added to the inftrumental angular 
diftance, 
