Biology Reference
In-Depth Information
with light adaptation, suggesting that only the rods, not the cones,
adapted to changes in light levels (Parinaud,
1885
, p. 823).
It appears that Parinaud arrived at his theory of rod and
cone functions by his study of 'héméralopie' (note that in French
'héméralopie' means night blindness, while in English 'hemera-
lopia' means day blindness). By this term, he meant night blindness
(in English 'nyctalopia') - a defect that reduces the sensitivity of
the eye markedly. He explained this defect with the assumption
that rhodopsin in the rods of the night blind was non-functioning
(Parinaud,
1881
). Thereby, the normal form and colour vision,
and the lack of sensitivity increase during dark adaptation of the
night blind could be accounted for. Moreover, he could explain the
excellent acuity and colour discrimination within the rod-free fovea
of humans and also the excellent acuity performance of pigeons and
hens that lacked rhodopsin.
Furthermore, his photochemical presumptions could easily
explain the change in size of the so-called photochromatic interval
(the intensity interval between light and colour thresholds) obtained
psychophysically in the extrafoveal retina in a dark-adapted state.
Thus, in accordance with his predictions, the photochromatic
interval was found to increase from the deep red to the blue part of
the spectrum (bleaching effect increased) and to be absent when the
measurements were made within the rod-free fovea.
Moreover, the deterioration of acuity performance observed when
a test field is moved away from the central fovea (a major problem for
Schultze) was accounted for by Parinaud with his assumptions that
cones were the receptors responsible for form discrimination and that
their number decreased with eccentricity (Parinaud,
1884a
, p. 242,
1885
, p. 822).
Finally, Parinaud (
1884b
) pointed out that his model also
provided explanations for several other important psychophys-
ical observations not adequately accounted for previously, e.g. the
Purkinje phenomenon (see Purkinje,
1825
) as well as difficulties
encountered by photometric measurements. He did not formulate
