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 
[197, 198. 
But by being keenly alert, it will be noticed that the gound of the 
image is distinctly brighter at the time when the character of it 
becomes confused than it is afterwards when the brightest places 
again appear outlined on a wholly black ground. In such cases, there¬ 
fore, it is not that the luminous impression has vanished and returned, 
but simply that the contrasts of brightness are temporarily more sub¬ 
dued, and the power of perceiving them has to be restored by new 
changes of colouring and brightness of the after-image. Moreover, with 
images that comprised a variety of objects of different degrees of 
brightness, the author’s invariable experience has been that the 
brighter a particular object was, the longer it took to disappear entirely 
from the positive image. The after-images which Aubert got by 
illuminating the object by electric sparks were probably faint; and yet 
his experience was that the positive after-images persisted longer 
when the sparks were feeble than when they were strong. 
On the other hand, if there has been any violent movement or 
pressure or shaking of the eye in uncovering and covering it, the 
result at first will be a confused light chaos, out of which then the 
after-image may gradually evolve. Similarly, an after-image already 
developed will be temporarily or entirely obliterated by pressure, 
movement, or trembling of the eye, or by external light. 
Supposing that the external light had acted only a very short time, 
and was not excessively brilliant, and that the field of view has been 
kept entirely free from all traces of external light, usually the positive 
image will disappear without passing into a negative one.1 But if 
while the positive after-image is still there, or just after it has dis¬ 
appeared, the eye is turned towards uniformly illuminated surfaces, 
or even with closed eyelids is directed to bright surroundings, a negative 
after-image appears. The stronger the positive after-image, the 
stronger also must be the reacting light to transform it into a negative 
one. There is always a certain intensity of the reacting light for which 
the positive image simply disappears without becoming negative. 
If the reacting light is more intense than this, a negative image comes 
out ; if it is less intense, the image remains positive and simply becomes 
less distinct. Incidentally, as the intensity of the reacting light is 
increased, the distinctness of the after-image also becomes greater; 
until the intensity exceeds the degree that is best suited for discerning 
small fractional differences of luminosity, and then the image begins 
to get less distinct. In this way too after-images of fainter primary light 
can be obtained by using more intense reacting light, but they have 
1 Concerning the so-called Purkinje after-image following brief stimulation by light, 
see Nagel’s Appendix, I. B. 6. at end of this volume.—N.


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