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
103, 104.] 
§11. Blur Circles on the Retina 
The experiment may be varied in many ways. Go to a window, 
for example, and hold a needle vertically about six inches in front 
of the eye, so that it appears to cross the horizontal window-bar at 
right angles. If you look at the needle, you will also see the window- 
bar as an indistinct dark band, or if you look at the window-bar or 
at the view outside, you will see the needle simply as a blurred vertical 
band in the field of view. Again, look through a small hole at the 
more distant objects beyond, and you may see either those objects or 
the edge of the hole distinctly, but never both at the same time. The 
first form of the experiment, however, is the most striking and also 
the best calculated to make it clear that the phenomenon is not due 
to any change in the direction of vision. 
It is to be noticed, in all these cases, that while one may not see 
clearly two unequally distant objects at once, still they may be 
seen distinctly one at a time, and that the transition from one to the 
other is entirely under the control of the observer. 
This peculiar process whereby the eye is enabled to see distinctly 
objects at different distances is called accommodation or adaptation1 
of the eye to distance. 
When the object is far away, a very considerable change of distance 
will make but slight alteration in the position of its image. Thus, if 
an eye is accommodated for an infinite distance, the blur circles of 
object-points even as close as twelve metres say, are so tiny that the 
distinctness is not seriously impaired. But when the eye is accommo¬ 
dated for near vision, a slight change of distance one way or the other 
will cause the object to be entirely out of focus. That segment of the 
visual axis where, for a given state of accommodation, an object can 
be seen without being indistinct is what J. Czermak has called 
the “line of accommodation.” The length of this segment increases 
with its remoteness from the eye, and becomes infinite when its 
distance is very great. 
These effects may be easily verified by keeping the eyes fixed 
on a needle or similar object erected an inch or more in front of a 
printed page. If the observer moves his eye as near the needle as he 
can without being unable to see it distinctly, the page appears blurred; 
but if the observer now moves his eye away from the needle, still 
fixing it steadily, the printed page becomes clearer and clearer. 
The reason we are able to “sight,” and to tell whether two points 
at different distances are exactly in line with the eye, is just because 
the blur circle of a distant object is very small when the eye is accom¬ 
modated for another distant object. Strictly speaking, only one of 
the points sighted can be seen distinctly at one time, while the others 
1 The latter term is no longer used in this sense. G.


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