( ) 
eafy to keep the Plane of the Inftrument fo near that 
of the before-mentioned great Circle as not to want 
any, if the Situation of that Circle be known: If it 
be not, the Obferver, when he fees the two Objects 
together, may turn the Inftrument on the Axis of the 
Telefcope, ’till he finds that Polition of it by which 
he obtains the leaft Angle:, and this (if the Specula 
are fet truly perpendicular to the Plane of the In- 
ftrument) will always happen when the Objeds ap- 
pear to coincide in the Line g h, as expreifed in the 
third Fig, of tfranf. No 410. 
In Page 152 of the fame, a Rule is given for find- 
ing to which Hand of the Obferver the Objed feen 
by Refledion ought to lie, but is reftrained to the par- 
ticular Form of the Inftrument there defcribed. The 
general Rule is, that when the Index is brought to the 
beginning of the Scale (i.e. to o° when the Inftru- 
ment is defigned for Angles under 90°, or to 90° when 
it is defigned for Angles from 90° to 180°) if then a 
Line be imagined to be drawn on it parallel to the 
Axis of the Telefcope, or Line of Diredion of the 
Sight, fo as to point towards the Object feen diredly } 
which ever way this Line is carried by the Motion of 
the Index along the Arch from o° towards 90° in the 
firftCafe, or from 90° towards 180 0 in the fecond,the 
fame way the Objed feen by Reflection ought to lie 
from that which is feen directly. 
Erratum, Pag. 154. Line 24, read, They may he 
either of Metals or Glafs Plates foil'd, having their 
two Surfaces ? 6cc. 
II. Ephe- 
