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
211, 212.] 
§23. Variations of Sensitivity 
eye fixed. The result is that the place on the retina where the image is 
formed of the central part of the sun is subjected more constantly and 
more intensely to the action of light than the part near the edge of the 
image. The sun itself is surrounded by the afterglow of the fight 
diffusely scattered in the atmosphere and in the eye itself. When the 
eye has been in the dark and is then suddenly exposed to the sun for a 
moment, the contour of the sun’s body can hardly be discerned in the 
dazzlingly luminous surface. Thus in these cases there is always a 
gradually diminishing action of fight from the centre towards the 
edge, and the result is a correspondingly different deportment of the 
separate phases of the after-image. The more intense the action, the 
more slowly do the individual phases proceed on the whole ; and hence 
at the edge of the after-image generally the earlier stages are seen 
gradually advancing towards the centre. Besides, owing to the slighter 
degree of fatigue, the sequence of colours out towards the edge is 
usually somewhat different from that in the centre. In keeping with 
this explanation, the after-image in its first stages has a greater extent 
than the apparent size of the sun, and it is natural to mistake the 
entire after-image for the image of the sun’s disc alone, and to suppose 
that the different coloured rings developed there belong to it, when 
in fact they correspond to its halo. The best way of getting a well 
developed after-image of the sun is to take a very dark coloured glass 
(or a smoked glass or a pile of several glasses of complementary 
colours) and look at the sun through it. Then it will appear simply as 
a dimly visible disc of fight. Now remove the glass for a moment, and 
close the eyes immediately. The action on them has been compara¬ 
tively slight, and they have not had much time to change their adjust¬ 
ment. Meanwhile, however, the after-image develops very brilliantly. 
And in this case the author can also see a nucleus in the after-image 
which is uniformly coloured all over and is about the size of the ap¬ 
parent disc of the sun. Therefore, the deviations on the border may 
be attributed to errors of refraction in the eye. 
Under these circumstances, in the environs of the sun’s image the 
phases of the after-image due to white objects that have been seen 
for an instant pass rapidly in review: positive blue, pink-red passing 
through yellow into negative dark green ; while the image of the sun 
itself in this first phase appears as a faded, more or less round white 
spot, which, at about the time when the ground has become pink-red, 
enters the second phase and is colored bright blue. The second phase 
usually passes rapidly into the third, by the blue becoming green 
first at the edge, then at the centre also, while a red-yellow border 
arises at the edge which is darker than the surrounding field, and on


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