I >W ' . - 'll / 
( ) 
turned a little on any Axis perpendicular to its Plane, 
the two Images will move along a Line parallel to 
g h , but keep the fame Pofition in refped of one ano- 
ther ; fo that in whatever Part of that Line they be 
obferved, the Accuracy of the Obfervation will be no 
otherwife affe&ed than by the Indiftindnefs of the 
Objeds* If the two Objeds be not in the Plane of 
the Inftrument, but equally elevated on, or deprefs’d 
below it, they will appear together at a Diftance from 
the Lin t gh y when the Index marks an Angle fome- 
thing greater than their neareft Diftance in a great 
Circle : And the Error of the Obfervation will in- 
creafe nearly in Proportion to the Square of their 
Diftance from. that Line; but may be corrected by 
help of the fifth Corollary. Suppofe the Hairs a e and 
bd , each at a Diftance from the Line g h , equal to 
4#£t of the focal Length of the Object-Glafs, fo as 
to comprehend between them the Image of an Objeft, 
whofe Breadth to the naked Eye is a little more than 
i ; and let the Images of the Objefts appear united 
at either of thofe Hairs : Then as the Sine Comple- 
ment of half the Degrees and Minutes mark’d by the 
Index, is to the doubled Sine of the fame ; fo is one 
Minute to the Error w 7 hich is always to be fubftrad- 
ed from the Obfervation. Other Hairs may alfo be 
plac’d in the Area abed ej\ parallel to g h , and at 
Diftances from it proportional to the fquare Roots- of 
the Numbers i, 2, 3, 4, £fc, and then the Errors to 
be fubftrafted from the fame Obfervation made at 
each of thofe Hairs refpe&ively, will be in Propor- 
tion to the Numbers 1, 2, 3, 4, &V. This Correc- 
tipn will- always be exaft enough if the Obferver take 
care. 
