384
The Sensations of Vision
[323, 324. N.
would be of interest especially in those cases in which one eye is dis¬
tinctly héméralopie, the other eye not being yet so or only in slight
measure. Of course, in this case the plan would be to make the sub¬
jective brightness of the light-stimuli equal for both eyes.
Although the facts here presented as to the light-sense and colour-
sense of the night-blind point to the correctness of Parinaud’s hypoth¬
esis, besides many other facts concerning the pathogenesis of the
condition of hemeralopia, which cannot be discussed here, but which
point in the same direction; Hess1 has come out recently as vigorously
objecting to the use of the results found in night blindness as arguments
in favour of the duplicity theory. He maintains that the night-blind
persons examined by him were able to perceive the Purkinje phe¬
nomenon; that after dark adaptation they exhibited less sensitivity to
light in the centre of the retina than in the periphery; that red pigments
in dim light were visible to them without colour, etc. These results are
in keeping with some which the writer could instance concerning
various night-blind subjects. On the basis of such observations, Hess
concludes that Parinaud’s hypothesis of the origin of hemeralopia
as being due to the disappearance of visual purple is thus upset. But
the hypothesis does not imply that everybody that is night-blind is
entirely without visual purple and the twilight mechanism in v. Kries’s
sense. Nobody seriously thinks this. Night blindness is a symptom of
a number of ocular diseases and occurs in the most various degrees. In
many cases it is progressive, and consequently it is not surprising that
in mild and medium degrees of it the effects mentioned by Hess should
have been obtained. These effects are explained by v. Kries, Pari¬
naud, the writer and others as being the expression of a participation of
the twilight mechanism in vision. The process of dark adaptation by
which the twilight mechanism is gradually inserted along with the
daylight mechanism and made to function is merely accomplished far
more slowly in the night-blind patient than in the case of a person
with normal vision ; and the Purkinje phenomenon takes a longer time
to occur and eventually is fainter, frequently with almost no traces of
it. Moreover, in night blindness the sensitivity to light in darkness
does not cease increasing, but it merely proceeds more slowly and to a
less extent. All that Hess’s experiments show is that certain qualita¬
tive changes in colour vision that go hand in hand with adaptation may
occur in spite of the existence of night blindness. This has never been
denied. The views which are here advocated would not be affected
even if it can be shown (which incidentally Hess has not done) that
the cone mechanism in night blindness is sometimes or always impaired
1 C. Hess, Untersuchungen über Hemeralopie. Arch. /. Augenheilk. LXII. 1908.