Formula. 
Ref. index of medium = /u .. 
Ref. index of lens = n'. 
Radius = 1. 
Variation 
of /j. when 
/*'=£• 
A 
1 
/* = ^ - y 
1 - 1-25 
B 
^ “ 1 + Y 
1-25 - 1*5 
C 
1-5 - 1-75 
D 
1-75 - 2 
E 
fX = 3 fX 
2 - 2-25 
F 
2-25 - 2-5 
G 
fX 
= 4 / 
2-5 - 2-75 
H 
2*75 - 3 
Variation 
of Focus 
in Inches. 
\ 
2-4 
\) 
