( 154 ) 
care (efpccially when the Angle comes near iBo°) to 
keep the Plane of the Inftrument from varying too 
much from the great Circle paffing thro 9 the Objefts. 
In regard to the Workmanftiip, if an Exaftnefs be 
required in the Obfervations, the Arch ought to be di- 
vided with the greateft Care ^ becaufe all Errors com- 
mitted in the Divifion are doubled by the Reflections. 
The Index muft have a fteady Motion on the Cen- 
ter, fo that the Axis of it remain always perpendicular 
to the Plane of the QCtant ; for if that alter, it will 
be liable to vary the Inclination of the Speculum it 
carries to the other : The Motion muft likewife be 
eafy, left the Index be fubje£fc to bend edge- ways: 
For the fame reafon it fhould be as broad at that End 
next the Center as conveniently can be. The Specula 
Ihould have their Surfaces of a true flat j becaufe a 
Curvature in either of them, befide rendering the Ob- 
ject indiftinct, will vary its Pofition, when feen by 
Reflection from different Parts of them: They muft 
alfo be of a fufScient Length and Breadth for the Te- 
lefcope to take in a convenient Angle without lofing 
the Ufe of any Part of the Aperture of its ObjeCt- 
Glafs, and that in all the different Pofitions of the 
Index. They may be either of Metal or Glafs Plates 
foil’d, having their two Surfaces as nearly parallel as 
they can ; yet a fmall Deviation may be allowed , 
provided either their thickeft or thinneft Edges (and 
confequently the common SeClion of their Surfaces) 
be parallel to the Plane of the OEtant: For in that 
Cafe, though there arefeveral Reprefentations of the 
ObjeCt, they will be always very near one another 
in a Line parallel to c j\ and any of them may be 
ufed. 
