of the Eye-pieces of Telescopes. 27 



nr> r + s.x-r .a-b\'' a 

 RD=x , . — 



x + s .a + t\' .Ir 2 



2 ( \na-o(a+r) nr V o' V a // a; + s.a+r-.6 ' 



Call this A"- Y — . Then (in the same manner as in a former Pro- 

 position) 



2 



But RC= ^ 



x — r a-b __ \r x/ \b a' 

 Vx s/ Vr a/ 



s+x ' a -i-r ' b 



\r x) \B a) 



or = — — a ; 



\x s/ \r a) 



which, observing that - = -^^ ^ nearly. 



X nr nA •' 



1 n-\ 

 or = 



1 W-l / . 1 1 l\ lv>y 



.. GR = a- 



D 2 



