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1 ines on the plate, being no lefs than /even inches, 
thofe wires may be fuppofed to differ infenfibly from 
a rig it angle to each other. The telefcope beincr 
adjulled for feeing them diftindtly, I brought thal 
wire of the telelcope, which in celeftial obferva- 
tions reprefented a parallel of declination, to be ex- 
actly parallel to one of the wires on the plate, with 
the fmalleff interval pofiible; and, at the fame time, 
made the middle perpendicular, or horary wire, to 
pafs through the interfeftion of both the wires* in 
the window: when I plainly difcerned, that the wires 
of the telefcope were not exadtly perpendicular to 
each other, the fuperior angle to the right being ma- 
nifestly acute, and the fuperior one to the left obtufe. 
This I further verified by applying the acute angle 
to the left hand fuperior angle of the plate, turnin'* 
the wires in the telefcope a quarter round, from right 
to left, by the lerew adapted for this purpofe, when 
the fame difference appeared as before. This proved 
alio that the wires on the plate made exadt right 
angles with each other; otherwile the acute angle 
of the wires of the telefcope could not have ap- 
peared to differ equally from both of them. To 
find the exact difference of the angle made by the 
wires from a right angle, I had a third wire placed 
exadtly parallel to one of the former on the plate at 
the distance of ^!_th of an inch ; when by applying 
the angle of the wires of the telefcope to the right 
angle on the plate, the deviation of the former from 
the latter appeared to be equal to half the interval 
of the parallel wires at the extremity of the field 
of view ; but the femi-diameter of the field of the 
leleicope at the diltance of the wires in the window 
being 
