$ (abe—bep—a+b.cq—b-+-c.ar+-cpq+6+¢.pr+a+b+ c.qr — pqr) 
+a+6+c.qr+bps+a+b.qs + ars— pqr — pgs— prs— rs 
abc— bep—a-+b.cq— b-+-c.ar—abs + cpq+-b-Fe.pr 
Also the distance from the center of the lens at which the axis 
of a pencil is incident on it is 
- ab—bp-—at+b.qt+pq t a) nae 
Pq c—t pqr 
x {abc—bep-a+b.cq-b+c.ar+cpg+bh+c.pr+at+b+c.qr—pqr}. 
Hence the tangent of the visual angle, found by dividing this 
by », is equal to 
abe bev a--bse-a.b+e abn? te rig dade 
prs pqs pr rs qs” ps 
at+tb+ec 
+— 4+ + — 
Onl > Bi. oe Bait jaguoin) 
Making its variation=0 as before, and reducing, we have the 
general equation of the eye-piece with four glasses, 
4abc—3bcp—3.a+b.cq—3a.b+c.r—3abs+2cpg+2.b+e.pr 
+2.a+b+c.qr+2bps+2.a+6.9qs+2ars—pqr—pqs—prs—qrs=0 
If a, b, p, q, 7, 8, be given, 
_ (6.a+2qg—2a- p.2b—q) .(r+s)+rs (2a—p+q) 
~ p(3b—2.q+r)+q(3.a+6—2r)-a(4b—3r) ~ 
(20). In a common perspective-glass it was found, that 
a=) 5,9) aHen, (bi— 2.2), Cc = 1-8 i— tee g) = 1,8, = 1,8, s— 1,28 
The following numbers have been given for a good eye-piece: 
P=, gi — 21 re esi — a2 ae 23,00 — Ad cl AO, 
=v). 
