114 Mr. Arry on the use of Silvered Glass 
is the 
distance from the axis at which it is incident on the first eye-glass ; 
ip Lib lem olf obese Cho: bis by IE 
fb pbg—(p+) fg 
, poq=4 (Pee b(gtp)et(ptb+ gfe 
. —(p+b)f-bg+fg 
b : 
or m x pog—q (p+ b)f—o eee + OSE | = B) is the distance 
from the axis at which it is incident on the second eye-glass. 
Hence the tangent of the angle with the axis of the telescope 
2 poner e (by similar triangles) or m x ae b)f 
pbg—q(p+5)f—b+p)g—pok+(p+b +a) fet (p+o) fktbgk—-fek (_¢ 
m x bfgk (2) 
(15). Now by (12), we must have 6C=0, or (since m, b, p, q, 
are eee 
pbq +24 q(p+b) (og , ok + Mer) 
TH f+ if gk Cae (CFs 
ri ptb+q sk pth sg a se 
PD fg ie) _rttieg th poh te 77° 
But ¢=——> *=——. where h and / are independent of n: 
n 
ea pa ee eer ae ee tes eee ee 
, ce. ie ee eae Daily ic np +2n—1p; 
- 4 —2f(p'+p) on. Substituting and dividing by 5x we get at 
length 
g 
2Qpb — — = b,— —_ = b a 
(4--2.pF+2 bp.n—1.p +7) a+ (-5 494-2 bp.n—1 Pp et (1-7-2601 +p) 4 
— : o 
pb Sh SS SS — = 
eA hae n—1.p—+p—2b.n—1.p'+p-g 
When & has this value, objects near the edge of the field of view 
will not be coloured. 
