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
199, 200.] 
§23. Variations of Sensitivity 
From the phenomena thus far described certain things can be 
inferred as to the state of the part of the retina and the corresponding 
part of the nervous mechanism of vision which have been stimulated 
by the primary light. In the first place we find that the state of stimu¬ 
lation persists there for a long time still after extinction of the primary 
light, as is indicated by the positive after-images; and, secondly, that 
the nervous substance in question is less sensitive to new reacting light 
falling on it than the rest of the retina that was not previously stimu¬ 
lated. Thus, after light has acted on the eye, (1) stimulation keeps right on, 
and (2) the sensitivity to new stimuli is lowered. That stimulation leaves 
behind a condition of lowered sensitivity for stimuli, is something 
that takes place also in the motor nerves and in other sensory nerves. 
Such a condition is called fatigue.1 
With increased intensity of the reacting light the negative after- 
images go on getting more distinct, until the intensity reaches the point 
where small percentage reductions of it can be perceived best; and 
hence we infer that fatigue of the nervous substance affects the sensa¬ 
tion of new light about in the same way as if the objective intensity 
of this light were reduced by a definite fraction of its amount. There is 
a lack of sufficient metrical data here, and hence in what follows all 
that will be attempted will be to indicate the general process of the 
intensity of the sensation of a fatigued portion of the retina con¬ 
sidered as a function of the intensity of the reacting light. As long as 
the positive image still exists along with the negative, the stimulation 
of the retina is compounded of the stimulation due to the primary light 
which still continues and the stimulation due to the reacting light 
which has been diminished by fatigue. In this sense we can regard the 
brightness of the after-image as the sum of the brightness of the posi¬ 
tive image and the brightness of the reacting light reduced by fatigue. 
Now if the diminution of the brightness of the reacting light is greater 
than the brightness of the positive image, the entire brightness of the 
after-image will be less than the brightness of the reacting light, as 
it looks to the unfatigued portions of the retina. And so the after- 
image will become negative. This is what happens when the reacting 
light is brighter. But when it is less bright, the brightness of the 
positive image is more than sufficient to make up for the loss by fatigue; 
and the image is positive. 
Let H denote the apparent brightness of the reacting light in the 
unfatigued parts of the retina, and aH in the part that has been 
fatigued, where a <1; and let I denote the apparent brightness of the 
1 Concerning more modern investigations as to the sensitivity of the retina resulting 
from “adaptation” to darkness or to brightness. See Nagel’s Appendix I. A.—N.


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