Let D F and C G reprefent the two Lens’s put 
together as before, having their common Axis in 
the Line E L, and B N a plane Speculum to which 
that Line is inclined in the Angle G H N, and let 
AB 
are Proportionals. But as F D is to D G, fo is the 
Tangent of the Angle I G D or KGE to the 
Tangent of the Angle I F D ; and as E H is to 
EG, fo is the Tangent of the Angle K G E 
to the Tangent of the Angle K H E. The Tan- 
gent of the Angle KGE therefore has the lame 
Proportion to the Tangents of each of the Angles 
I F D and K H E, and confequently thofe Angles 
are equal, ®- 
N. B. In the Demonftration of the above-cited 
Propofition of Huygens , the Thicknefs of the 
Lens’s are ncgle&ed, and the Diftance of the Points 
I and K, from the Line F G, fuppoled very fmall \ 
fo that if either of thofe are too great, there may 
arife a fenfible Difference between the Angles I F D 
and K H E. 
