134 Mr. jeaurat’s Account of 
lens c (namely, the diftance <p) have this condition, 
^x'BD-Bcx cd, viz. 3dly, itpx v-f—xy. 
From thefe 
three conditi- ^ 
ons, viz. 
' x+y=2x f -f, we eafily and incon- 
, fy=Fx, ^ teftably find what 
1 3° 2<p x F-f=xy, _ follows: 
T$C—X~ —— F — - the dift. from the focus b to the lens ch, 
F+/ 
2 fx FT/T the dift. of the relative focus- d, with re- 
■- y f+/ { fpedt to the two lenfes a and c, 
<p - 2F ^ X r ~f- the focal diftance of the lens c. 
This folution juft found is general; but to adapt it to 
a particular cafe, which may be proper for practice, I 
fhall inveftigate what relation ought to take place be- 
tween the diftances x and / when q> is = /. This fuppo- 
fition 
ie foyer de la lentilie c (favoir le foyer <p) ait cette condition ci <p X bd ~ bc x cd 
fa voir, 3 0 , 2£>(f —f) scy% 
Avec ces trois con- 
ditions, favoir, 
? 
x+y-z (f-/), 
fy — Fx, 
Z<p(t'-/)=xy, 
Ion trouve facile et inconteftable- 
| ment ce qui fuit. 
bc~x — £} diftance du foyer b a la lentilie ch. 
F-f/ 7 
f) 
CDi-y = — F ' I j ^yer relatif d, a l’cgard des deux lentilles a et c* 
2F/Tf— /) 
<? = 
( F +/) 
foyer de la lentilie c. 
‘Cette foiuti’on qu 9 on vientrde trouver eft generate* Mais afn d’adopter uft 
'£3*5 .particular, qui foit commode a pratiquer, je vais chercher les rapports, que 
do i vent 
