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
219, 220.] 
§23. Variations of Sensitivity 
where the negative after-image is developed is less sensitive to external 
light than the unfatigued retina. Thus the persistence of stimulation 
and the lowering of sensitivity is not an hypothesis but a direct con¬ 
sequence of facts. Moreover, these two circumstances suffice to explain 
by far the greatest number of the more obvious and constant phenom¬ 
ena in this region, especially the phenomena of the altered intensity 
of light and of positive homochromatic and negative complementary 
after-images. To bring the very intricate phenomena of colour waning 
of intense or persistent luminous impressions entirely within a simple 
scheme, would probably be a very difficult task indeed at present and 
would involve all sorts of arbitrary assumptions. However, we can 
inquire why these phenomena must be so variable. We really know 
neither the law by which a more or less advanced fatigue of the eye for 
a single colour disappears, nor how far the after-effect of light depends 
on fatigue. In Fechner’s opinion negative complementary images in 
the dark field are to be regarded as new modes of response to the 
internal stimuli of the retina. On the other hand, many physicists 
have looked on these images as effects of a new antagonistic activity 
of the retina. Plateau1 in particular, has elaborated this view into 
a consistent theory. He demonstrated that the same complementary 
coloured images may be seen even without any external light at all ; and 
as he was not then cognizant of the intrinsic fight of the eye, he knew 
no other way of explaining the phenomenon except as a new antagonistic 
activity of the retina. Moreover, having noticed also subsequent 
variations of the positive and negative images, he advanced the propo¬ 
sition that whenever the retina was strongly stimulated by light, it 
experienced a series of oscillations, before it came to rest, which in¬ 
volved its passing successsively and alternately through opposite 
stages. These antagonistic conditions would correspond to the sensa¬ 
tion of complementary colours. He connected this with certain con¬ 
trast phenomena, which will be considered more directly in the next 
section; and assumed for the spatial extension of the impression a 
series of such oscillations. On the other hand, it must be remembered 
that negative complementary after-images do not consist in a lively 
activity of the retina, but, on the contrary, are rendered visible by being 
diminutions of the already existing sensation of intrinsic light; and 
that, moreover, as is almost always discovered on closer attention, those 
alternations between positive and negative images are dependent on 
external conditions, especially on slight changes in the illumination 
of the fundus of the eye. In the author’s opinion it is very hazardous 
to develop a theory on the basis of such delicate and extremely un- 
1 Ann de Chim et de Phys. LUI. 386.—Poggendorfs Ann. XXXII. 543.


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