Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 1. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
148, 149.] 
§13. Chromatic Aberration in the Eye 
from behind by a lamp whose rays after traversing the glass 
and the hole in the screen enter the observer’s eye. Under these 
circumstances the aperture may be regarded as a luminous point 
emitting red and violet rays; which will appear differently, depending 
on the distance for which the observer’s eye is accommodated. When 
it is accommodated for the red rays, the violet light produces a blur 
circle, and the opening appears as a red point with a violet fringe 
around it ; and vice versa. There is an intermediate state of accommoda¬ 
tion for which the focus of the violet rays is in front of the retina, and 
that of the red rays beyond it, in such fashion that the coloured blur 
circles on the retina are equal in size; and only when this is the case 
will the source appear to be of the same uniform colour. For this state 
of accommodation, there might be some light of intermediate colour, 
say, green, which would be focused at a point on the retina. 
Incidentally, this method affords a means of finding with consider¬ 
able degree of accuracy the distances for which the eye can be accom¬ 
modated for the intermediate parts of the spectrum; for they are the 
same as the distances for which the eye can see the mixed red-violet 
spot as being of uniform colour. The difference of colour at the edges 
is very easily detected, even by an unpractised observer; and it is 
much easier to distinguish than the inexactness of a white image. 
When the eye is accommodated for light of a certain definite frequency 
for a distance farther than that of the luminous point, the blur circle 
of the red rays on the retina will be larger than that of the violet, and 
the spot will appear as a violet disc with a red border; whereas the 
reverse effect will be observed when the eye is accommodated for a 
distance less than that of the luminous point. Effects similar to these 
always occur whenever an object emits two kinds of light of very 
different frequencies. The phenomena are very striking, for example, 
in certain experiments on the mixing of spectrum colours which will be 
subsequently described in the theory of colour mixtures. 
With white light there is, of course, also some separation of the 
component colours, but ordinarily it is hardly noticeable. In this 
connection, experience shows that a Avhite surface which is farther away 
than the point of accommodation of the eye, is tinged with a faint blue 
border; whereas if the object is nearer than the point of accommoda¬ 
tion, the border will be a faint reddish yellow. But when the eye is 
accommodated for the exact distance of the white surface, no such 
coloured border is seen, unless some opaque obstacle is held close to 
the eye so as to cover part of the pupil; in which case a coloured border 
appears along the opaque edge. The border between a white and black 
field appears yellow when a card is interposed halfway in front of the


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