[ ] 
%e a very little augmented, rather than othcrwiie ; 
wherefore we put it here = .oyd^g. 
The Radius of the concave Surface of the Lens, 
turn’d towards the Speculum, of 
=r2.8 Inches. 
The Radius of the concave Surface of the Lens, 
turn’d from the Speculum, njiz. of KLM,^ 6 .j 
Inches. 
The Thicknefs of the Lens at the Vertex LHz=z ~ 
of an Inch. 
The Aperture of the Lens muft be about -g- of the 
Aperture of the Speculum. 
HR, the Diftance of the Focal Point R from the 
Point H, where the abovefaid Lens is to be placed, 
fo as to corred the Errors arifing from the different 
Refrangibility of the Rays, and alfo the Errors of 
the fpherical Figure, = 2.|-| Inches. 
HR, the Diftance of //the Vertex of the Lens from 
R the Focus of the Telefcope, =6.8 Inches. 
And if we fuppofe the Diameter of the Pupil of the 
Eye to be f of an Inch, (though it has not one 
certain Meafure) then the Diameter of the greateft 
Aperture of the Speculum, that can ever be of Ufe, 
will be 6-| Inches, nearly. 
The fmall plano-convex Eye-glafs O ^muft always 
have one common Focus with the Telefcope, to 
wit, the Point R tranflated to r, by Refledion from 
the Bafe of the Prifm N ; for which Reafon it muft 
retain, at all times, an equal and invariable Diftance 
from the Lens GHIKLM 5 which Diftance will be 
the Focal Length of the faid Eye-Glafs more HR 
(= HN -\- Nr) the Diftance of the Lens from the 
Focus of the Telefcope R. 
The 
