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
Dioptries of the Eye 
[149, 150. 
pupil from the side of the black portion, and blue when it is inter¬ 
posed from the side of the white portion. 
The phenomena of chromatic aberration in the human eye as 
above described are easily explained by the fact that the second focal 
point for violet light lies in front of that for red. In Fig. 68 the lumin¬ 
ous point is supposed to be at A; the first principal plane of the eye is 
represented by the 
straight line bxb2 ; the 
points on the optical 
axis designated byr 
and v are the points 
conjugate to A with 
respect to the red 
and violet rays, respectively; the straight line cc represents the plane 
where the outside red rays of the cone bib2r intersect the outside 
violet rays of the cone byb2v. Obviously, if the retina of the eye is 
in front of the plane cc, that is, when the eye is accommodated for 
an object farther away than A, it will be illuminated at the boundary 
of the cone by red light only, whereas near the axis there will be a 
mixture of colours. When the retina coincides with cc, so that the 
eye is accommodated for light of intermediate frequency coming from 
A, it will be illuminated all over by a uniform mixture of colours. 
And when the retina is beyond cc, as will be the case when the accom¬ 
modation is for a point nearer than A, the edge of the illuminated 
area will be violet while the central part will be a mixture of colours. 
Now suppose that when the eye is accommodated for A, so that the 
retina is at cc, the lower half of the aperture bA>2 is covered by an 
opaque screen; thereby cutting off all the violet rays beyond b2v and 
fv, all the red rays between b2r and /r, and, of course, also, the pro¬ 
longations of these rays. All the violet illumination disappears then 
from the part of the retina above the axis, and all the red from the part 
below the axis, the result being a small blur circle on the retina which 
is red above and violet below, instead of a clear-cut image of the 
point A. 
If the source at A, instead of being a single luminous point, is a 
surface uniformly illuminated with both red and violet light, there will 
be formed on the retina simultaneously both a red and a violet image 
of this surface, one of which is necessarily blurred. But the blurred 
image of a uniform surface looks exactly the same as if it were in 
focus, except towards the edge where the blur circles of the separate 
points only partially overlap one another, and here the image fades out, 
encroaching on the surrounding darkness only so far as the blur 
circles of the border points extend. Now if red and violet images of a 


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