Contribution of S. Hecht - Duplicity Theory of Vision - page 137

first period was found to be of very short duration representing the
time occupied by the necessary exposure to light. To test his assump-
tion that this initial reaction of the light reflex was photochemical
in nature, he investigated whether the sensitization process of the
siphon reflex had the characteristics of a photochemical reaction. He
measured the minimum light intensity necessary for a response to
take place. In each run exposure time remained constant, but could
be varied between runs.
Two quite different photochemical characteristics were investi-
gated: (1) the well-known Bunsen-Roscoe reciprocity law where the
quantity of light energy necessary to produce a given chemical effect
is constant whether the intensity is low and the exposure long, or
the reverse, and (2) the low temperature coefficient of photochemical
reactions.
The results showed that the sensitization process of the siphon
reflex of Mya followed the photochemical laws, and Hecht therefore
concluded that it depended on a photochemical reaction.
17.3 Supporting evidence obtained from
psychophysical experiments
Hecht, then, turned his attention to the adaptation processes of the
human eye to explore whether his model also could be applied to
human vision. At first, he found it necessary to supply the psycho-
physical dark-adaptation data already available with more accurate
measurements. In particular, there was a need for reliable data on the
rapid recovery of sensitivity during the first few seconds of cone dark
adaptation. Just the extreme rapidity of recovery had made it difficult
to obtain accurate measurements. In an important paper, however,
Hecht ( 1921 /1922) managed to make highly reliable measurements
of the cone dark adaptation from the seventh second onward to the
complete dark-adapted state. In fact, some less accurate measurements
of the cone threshold were also obtained as early as two seconds of
dark adaptation and by extrapolation backwards he estimated the
threshold value at zero seconds dark adaptation.