Bauhaus-Universität Weimar

Titel:
Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition
Person:
Helmholtz, Hermann von
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit39650/353/
340 
The Sensations of Vision 
[287. N. 
Fechner1 stated that a bright surface viewed with both eyes does 
not look brighter than it does in monocular vision. Helmholtz 
corroborated this, and it is a well known fact so far as vision in a 
bright place is concerned. The only effect of closing one eye may be 
to make a faint shadow come over the surface. 
In performing this experiment, supposing that the initial adaptation is 
of that moderate kind customary indoors, it should be borne in mind that a 
certain degree of dark adaptation takes place very quickly in the eye that is 
closed or screened with the hand. When the eye is uncovered, it is not as if 
the free eye were reinforced by a second eye of equal sensitivity, but by an 
eye which at the instant when it begins to take part has increased sensitivity. 
As a matter of fact, when light adaptation is very good, and one of the eyes has 
been closed for a very short time, there is generally no appreciable difference 
of brightness between monocular and binocular vision. 
That there is a real difference between the pair of eyes for photopia 
and scotopia so far as the summation of binocular stimuli is concerned, 
has been proved by Piper. His method consisted in determining the 
monocular and binocular thresholds alternately throughout the entire 
test of the dark adaptation. The results of one of these tests, in which 
the writer acted as observer, are given in Table X; and graphically 
exhibited in Fig. 66. The experiment was made before the construction 
of the adaptometer; consequently, the absolute sensitivity values are 
not directly comparable with those given in other places, but are 
represented in terms of an arbitrary unit. 
Table X 
Binocular 
Right Eye 
Left Eye 
Time 
in min. 
Sensitivity 
Time 
in min. 
Sensitivity 
Time 
in min. 
Sensitivity 
0 
86 
34 
111 
1 
111 
3 34 
272 
434 
498 
5 
498 
sy2 
2724 
934 
2914 
1034 
3419 
14 34 
11815 
1534 
13521 
16 
14516 
20 34 
41649 
2134 
27778 
2234 
22957 
2734 
65746 
2834 
38447 
30 
33058 
37 
81632 
39 34 
40000 
4034 
36982 
5234 
59 
97656 
97656 
56 
40000 
57 
41649 
A distinct separation between the curves for monocular and bin¬ 
ocular vision is not manifest until after 14 minutes. Investigating 
various soldiers, who were employed as normal controls in an investiga¬ 
tion of hemeralopia, Messmer (loc. cit.) got quite similar results. 
1 Fechner, Über einige Verhältnisse des binokularen Sehens. Ahhandl. der sächs. Ge- 
sellsch. d. Wissensch. VII. 1860. S. 423. See also Bd. III. S. 424.
        

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