Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
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.


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