Relationship between amount of rhodopsin and sensitivity during dark adaptation - Duplicity Theory of Vision - page 150

( 1885 ) was quite inadequate as an explanatory basis of dark and light
adaptation of the visual system. Hence, a new theory was called for.
To meet this need, Wald ( 1954 ) proposed his famous compartment
theory of visual sensitivity regulation. The theory was intended to
cover both light and dark adaptation for both rods and cones.
To explain the large rise of threshold when only small amounts
of rhodopsin was bleached, he presumed that: (1) the rod receptor
was a compartmented structure, (2) each compartment contained a
considerable quantity of rhodopsin molecules, any one of which, on
absorbing a quantum of light, would discharge the whole contribu-
tion of the compartment when the eye was in a dark-adapted state,
and (3) the compartment could not contribute again until all of its
rhodopsin molecules were restored.
Based on these presumptions Wald ( 1954 ) described the rise of
threshold intensity by the formula:
n 0 / ( n 0 - n x )
where n 0 represented number of compartments in the rods and n x the
number of compartments that had absorbed at least one quantum of
light. Hence, absorption of the first quantum of light by dark-adapted
rods would raise the threshold n 0 / ( n 0 - 1) times. When half of the
compartments had been discharged, the threshold would be doubled,
i.e. [ n 0 / ( n 0 - ½ n 0 )], when 9/10 had been discharged, the threshold
would have been raised ten times, i.e. [ n 0 / ( n 0 - 9/10 n 0 )], and so on.
Wald ( 1954 ) held that his compartment theory could
explain both the discrepancy between the rise in threshold and
concentration of rhodopsin and the delay of sensitivity increase
during the dark-adaptation period obtained by increasing the
proceeding light-adaptation level. Thus, the theory presumed that
when one molecule in a compartment had absorbed a light quantum
and discharged it, absorption of more light quanta would not raise
the threshold further, but would tend to retard the subsequent dark
adaptation, since the compartment could not influence sensitivity
until its last rhodopsin molecule had been regenerated.