Jor the Mirrors of Reflecting Telescopes. 115 
C 1 , Lee Fe , a sarees 
(16). Since p= Ine +2.H—-1.p> 2p +2.n—1.p+¢, we may 
change this expression into the following: 
“be apb.p' =a G- 2b )g 
which is rather more convenient for practical application. 
(17). To construct a telescope on these principles it will be 
most convenient to assume the values of F anda; Db will generally 
be rather greater than the distance of the two mirrors; and / will 
be found by the equation D + hes? 
fi To determine p, »’, r, R, 
we have then the four equations 
~— 1 
2np+2.n—-1.p=- 7 
p ra -+(7) 
2nR+2.n—1.r peels 
a’ (p+p')+F° (r+ R)=0.. (10) 
-1 § —— ,(n—1. ‘— D)? 
at {7 (0+ Dy’ .(ptntiD)+2p Cm betnp ~D) 
+n.n—1. (2p ee aren a (2.041 Pert il ste p-"** v)} 
4+ FF om 1 yap (al .r+nR) 
n 
+n.n—i (2R+ r) (2 Pres et seg #1} =0....(11). 
From the three first equations 
ld etd 
p= ont he 
