change in back focal distance 
second focal length at n, 
desired water refractive index 
z actual water refractive index 
desired nodal object distance 
actual nodal object distance 
distance from first vertex to first nodal point 
for a water refractive index of n4. 
distance from first vertex to first nodal point 
for a water refractive index of n, . 
radius of first lens surface in contact with the 
water. 
Table 1 of Tech Memo No. 15 dated 29 August 1975 is a tabula- 
tion of the back focal distances of the C201 ELCAN water con- 
tact lens for nodal object distances for 7 feet and infinity and e 
for water refractive indexes from 1.333 to 1.353. By scanning 
Table 1 and with a few simple computations, it can be demon- 
strated that the change in back focal distance in millimeters, 
for all practical purposes, is equal to 12.33 times the change 
in the water refractive index for nodal object distances from 
7 feet to infinity. This approximate but simple linear rela- 
tionship can be substituted for the precise but more involved 
Eq. 18 with no significant error for the intended application. 
Therefore, the following approximate equation is presented for 
the C201 lens: 
{6 = 12.33 Eq. 19 
where 
On = change in water refractive index. 
Substituting Eq. 19 in Eq. 17 for a low contrast target and 
f-number of 4, 
& . (0.625) (4) 
12.35 
R = — Ea. 20 
5m 
i 
SR’ 
or 
An = Eq. 21 
Assume that a limiting resolution of 50 cycles per mm is per- 
missible for the change in back focal distance. Substituting 
a value of 50 for R in Eq. ?1l, 
a 1 
On = Tovey 
An 0.004,