( >8<S ) 
LEMMA 
Let the Line FG be the commonAxis of the two 
Lens’s I D and KE, of equal focalLengths ; to which 
let the Lines AD, D B and B E, be each equal ; and 
let a Ray of Light, ifluing from a Point in the 
Axis F, fall on the Lens I D at I, and be there re- 
fracted into the Line I G, cutting the Axis in G, 
and meeting the Lens K E in K, where let the Ray 
be again refraCted into the Line K H, cutting the a- 
forefaid Axis in H : The Angles I F D and K H E 
are very nearly equal. 
Demonstration. 
t ■ ■_£*•; Lva ','IU tn • KjiXiiOL:* l *«# T>i to ? ! f r i > ’ J 
It is known from Doptricks, that the Lines F I, 
I G, K H, and F G, are all in the fame Plane ; 
and by the ConftruCtion the Lines A D, D B, and 
B E are equal ; and by Tropof. zo of Huygens’* 
c Dioptricks^ the Lines F A, F D, and F G are con- 
tinually proportional ; and confequently F A is to 
A D as F D to D G, and dividing, F A is to A D as 
F D — FA (— to A D) is to D G — AD (= to 
B G.) Therefore A D is to B G as F D to D G. 
By the lame ‘Propojltion, the Lines BG, E G, and 
H G are alfo continually proportional, and B E 
(= to A D) is to B G as E H is to E G. Hence 
it follows, that the Lines F D, D G, and EH, EG, 
are 
