an Tconantidiptic Telefcope. 1-3*3 
BC = X, 
CD=y, 
<p the focal diftance of the lens c. 
{ BD = AD-i-AB = 2 xF-f. 
BD^BC 4- CD — X+y. 
The two values of bd evidently give, ift, x+y-o. x f -f. 
That the image b,. given by the lens a , be feen at the 
diftance bc: and that the direction of the ray bhd may 
form a relative focus in n, whofe diftance may be equi- 
valent to ad, it is neceftary that «bx cd=ad x bc, namely,, 
id\j , fy=v x . 
That the object b, feen in the direction bh, may form 
a focus in d, it is neceftary that the focal diftance of the 
lens. 
BC 
CD ~y, 
„ <p foyer de la lentille c„. 
r'BDZZAD + ■AB = 2 (f— ■ /), 
\bd = bc +CD —X+yi 
Les* deux valeurs de bd donnent evidement, 1°, x +y~ 2 (f f ) , . 
Pour que I’image b, donnee par la lentille a , foit vue a la diftance Be: ett 
pour que la direction du raion bhd forme un foyer relatif en d, dont le 
foyer equivalent foit ad; il faut (voyez l’optique de smith dela Tradu&ion de 
l’Abbe rochoNj p. 278. art. 245.) dis-je que aB x cdzad x bc favoir, 2 0 , 
fy= vx - 
Pour que l’objet b vu felon la dire&ion bh, forme un foyer en d, il faut que. 
It: 
