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.