Bauhaus-Universität Weimar

Titel:
Helmholtz's treatise on physiological optics. Volume 1. Edited by James P. C. Southall. Translated from the 3rd German edition
Person:
Helmholtz, Hermann von
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit39649/148/
124 
Dioptries of the Eye 
[104. 
appear more or less blurred; and the exact alignment of two points 
may be obtained by simply getting the sharp image of one point into 
apparent coincidence with the centre of the blur circle of the other. 
A line passing through two such apparently coincident points is called 
a line of sight. All lines of sight intersect at one point within the eye, 
namely, at the centre of the image of the pupil formed by the cornea, 
known as the point of intersection of the lines of sight.1 
That the change taking place during the process of accommodation 
is an actual alteration of the optical image itself, and not simply a 
mode of sensation of the retina, as some physiologists have supposed, 
may be proved in the most convincing manner by the use of the 
ophthalmoscopè. With this instrument, which will be described in 
§16, the fundus of the eye can be seen distinctly, including the retina 
with its blood vessels and the images projected on it. If the patient’s 
eye is fixed on a given object, what we find is that the image of a light 
at the same distance away will be sharply focused on the retina, and 
at the same time the veins and other anatomical details of the retina 
will be clearly visible in the vicinity of this bright image. Now suppose 
the light is moved much nearer; its image will become indistinct, but 
the details of the retinal membrane remain as sharply defined as 
before. Attempts at seeing the changes of the retinal images in dead 
eyes from which the rear portions of the sclerotica and choroid have 
been removed or in the living eyes of white rabbits, whose sclerotica is 
nearly transparent, are both unsatisfactory and likely to fail, because 
the images thus observed are usually not exact enough to enable the 
investigator to recognize minute variations in them. Even in a living 
eye noticeable alterations of the image, due to accommodation, do not 
occur unless the object itself is comparatively small and precise 
like a thread or a printed word. Large objects may be still recognized 
by form even when the accommodation is not correct. 
But on the retina of a dead eye all the finer details are effaced, 
as will be seen by artificially magnifying the image until it looks to the 
observer as large as it would have seemed to the observed eye, when 
its retina was still sensitive. 
These accommodation phenomena and the varying positions of the 
ray-focus with respect to the retina are still better demonstrated 
by an experiment due to Scheiner. Two pinholes are made in a card 
at a distance apart less than the diameter of the pupil of the eye. 
With one eye closed, the observer looks through both holes at a small 
1 tThis point coincides, therefore, with the centre of the entrance-pupil of the eye. 
(J.^P. C. S.) H 
i Since the act of sighting requires central visual acuity, we are justified!in speaking 
only of one line of sight; which is the one along the ray that after refraction in the eye pro¬ 
ceeds to the fovea centralis. G.
        

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