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 
continues to see the bright image of the flame afterwards in the dark, 
surrounded by the rather fainter glow of the globe and the other 
portions. When the eye moves, the after-image moves in the same 
way, always occupying the place in the field of view that corresponds 
to the part of the retina on which the light fell originally. In order for 
the after-image to be sharply delineated, a single point on the object 
should be steadily focused. If the eye has wavered, the image will be 
faded, or perhaps there may be two or three images of the object partly 
overlapping. If the after-image is quite sharply delineated, details in it 
can be noticed under proper conditions that had not attracted atten¬ 
tion in gazing at the object itself and had therefore been overlooked. 
After-images of bright objects, in which the fight portions appear 
fight and the dark parts dark, are called positive after-images. As 
they gradually begin to vanish, they are usually mingled with other 
images, in which the fight parts of the object look dark, and the dark 
fight, so-called negative after-images. Apparently, these latter effects 
are due mainly to the fact that the sensitivity of the retina for fight 
has been altered also by its previous stimulation. The two kinds of 
phenomena cannot be kept strictly separate from each other in a de¬ 
scription of them. Accordingly, the more detailed description of both 
positive and negative after-images will be reserved for the next chapter; 
while this chapter will be devoted to describing the effects, of quickly 
recurrent fight stimuli, in which the persistence of the fight impression 
appears pure and simple, without being affected essentially by changed 
sensitivity of the retina. 
The leading fact here is that intermittent light stimuli of a uniform 
kind, occurring with sufficient rapidity, produce the same effect on the eye 
as continuous illumination. For this purpose, all that is necessary is 
that the repetition of the impression shall be fast enough for the after¬ 
effect of one impression not to have died down perceptibly before the 
next one comes. 
The easiest way to show this is with revolving discs. When a black 
disc with a bright white spot on it is rotated fast enough, a grey ring 
appears instead of the revolving spot. This ring looks everywhere 
perfectly uniform, and there is no longer any movement to be seen. 
As the eye looks steadily at some one place of the apparently stationary 
ring, the places of the retina on which the image of the ring is formed 
receive in swift repetition the image of the white spot that traverses 
the circle. They get, therefore, a fight impression which appears con¬ 
tinuous on account of the rapidity with which it is repeated. Of course, 
it is not as strong as it would be if continuous white fight fell on the 
retina; and so the ring looks grey instead of white. On the other hand, 
if the eye itself is moved, carrying the point of fixation around with


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