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The telefcope fhould be fixed by the inftrument- 
maker fo as to command a full field of view when 
the inftrument is placed at 90° if the inftrument be 
an odant, or 120° if it be a fextant; becaufe the 
index-glafs then ftands more oblique with refped to 
the incident and reflected rays, and confequently the 
field of view of the telefcope, as far as it depends 
upon the index-glafs, will be more contraded than 
in any Gther polition of the index: but if there is a 
fair field of view in this cafe, there neceffarily muff 
be fo in every other pofition of the index. 
The two parallel wires will be very ufeful on many 
occafions, as well in the fore as the back-obfervation. 
In taking the altitude of the Sun, Moon, or ftar, 
dired the fight towards the part of the horizon 
underneath, or oppofite to the objed, according as 
you intend to obferve by the fore or back obfer- 
vation, and hold the quadrant that the wires may 
conftantly appear perpendicular to the horizon, and 
move the index till you fee the objed come down 
towards the horizon in the fore-obfervation, or up to 
it in the back-obfervation, and turn the inftrument in 
order to bring the objed between the wires ; then 
move the index till the Sun or Moon’s limb, or the 
flar touch the horizon. The nearer the objed is 
brought to an imaginary line in the middle between 
the wires (it is indifferent what part of the line it is 
brought to) and the truer the wires are kept perpen- 
dicular to the horizon, the more exad will the ob- 
fervation be* In the fore-obfervation, the objed ap- 
pears in its real pofition ; but in the back-obfervation, 
the objed being brought through the zenith to the 
horizon, the real upper-limb will appear the lowed; 
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