Volltext: Helmholtz's treatise on physiological optics. Volume 1. Edited by James P. C. Southall. Translated from the 3rd German edition (1)

Ill, 112.] 
§11. Blur Circles on the Retina 
same print when it is only 2.7 inches from his eye. Another thing to be noted 
here is that the apparent size of an object is bigger when it is brought near the 
eye, so that, other things being equal, its details can be more clearly made out 
than when it is farther away. Thus, tiny objects which are hard to see are 
sometimes examined even nearer than the near point of the eye, because the 
larger apparent size may more than compensate for the slight indistinctness of 
the details, and it may be better seen than it could be with exact accommo¬ 
dation under a smaller visual angle. It is evident that in order to use this 
method for measuring the range of accommodation, we must take different 
objects for different distances, each of them being just large enough to be 
recognized distinctly when the eye is accommodated for that particular 
Porterfield1 was the first to propose using Scheiner’s experiment for 
measuring the range of accommodation distances, and designed an optometer 
on this principle, which was afterwards improved by Thomas Young.2 The 
latter uses a fine white thread stretched on the black surface of a graduated 
bar. One end of the thread is close to the eye, which looks along the thread 
through two small peepholes or slits in an opaque screen, and sees the thread 
as a pair of straight lines which intersect each other at the point for which the 
eye is accommodated. This point may be located without difficulty, and its 
distance from the eye is the distance of accommodation of the eye at that 
particular instant. The method may be modified by using any other small 
objects whose distances from the eye can be varied. The objects chosen for 
the experiment must be small enough to be just distinctly visible through the 
holes in the screen, as, for example, slender needles against a bright back¬ 
ground, or fine holes and slits in dark screens. The eye must be sure to look 
through both openings at once, otherwise errors are 
liable to be made. The field of view in these experi¬ 
ments is contracted to ihe comparatively large 
blurred images of the two apertures, which partly 
overlap each other as shown in Fig. 58, a and b. 
Double images of a needle, as indicated at g, can be 
seen only in the bright middle region c that is com¬ 
mon to both openings, but not on either side in any 
part of the field that corresponds to one aperture 
alone. In this latter region the image is always 
single, as at h; which is the reason why persons sometimes have difficulty in suc¬ 
ceeding with the experiments unless they have practised with the apparatus. 
Another similar method for finding the range of accommodation and espe¬ 
cially for ascertaining the far point is more accurate, in the writer’s opinion, 
than that of looking through two openings. Daylight or light from a candle 
flame is admitted through a small opening in a screen. To an eye not accom¬ 
modated for it a small point of light like this looks like a star with five or six 
rays (see §14 below), whereas, when the eye is properly accommodated, it 
appears as a fairly well defined bright spot even if it is not uniformly round. 
Now move a screen slowly from one side in front of the pupil; and what 
generally happens in this case is that the pattern of light that is seen begins to 
fade away on one side, and this occurs on the same side as that from which 
the screen is interposed, provided the eye is accommodated for a point nearer 
than the luminous point, but on the opposite side if the eye is focused for a 
point beyond the source of light. But if the eye is accommodated for the 
luminous point, the image gets uniformly darker all over, or else it is eclipsed 
1 An Essay concerning the Motions of our Eyes. Edinh. medical Essays. Vol. I. p. 423. 
IV. 185 (1747). 
2 Phil. Transactions. 1801 P. I. p. 34.


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