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
Dioptrics of the Eye 
[372, 373. G. 
that is, between the ruled lines on the aberroscope and the retina, it is 
true that the sign of this magnitude does depend simply on the sign 
of the aberration; and, consequently, there would be no objection to 
the conclusions derived from investigations with the aberroscope, 
provided the instrument was set up in the vitreous humor! However, 
there is another factor besides an intervening refracting surface between 
the lattice of lines and the screen; and this factor cannot be calculated 
in the optical system of the eye, because the laws concerning it are not 
known for heterogeneous media. But it does depend on the distance 
of the lattice; and when this distance is considerable, it is appreciably 
affected by the aberration of the lens of the aberroscope. Thus, the 
value of the distortion whose sign is found by the aberroscope contains 
two terms, one of which depends on the aberration; and the sign of this 
term changes when the image of the luminous point falls beyond 
the retina, whereas the sign of the other term does not change. Con¬ 
sequently, the sign of the aberration of the bundle of rays refracted 
in the eye can be obtained by the investigation with the aberroscope, 
provided the curvature of the shadows changes sign according as the 
image of the luminous point falls on one side of the retina or on the 
other. Now, as a matter of fact, this does happen as a rule with the 
normal eye, and therefore the normal positive aberration may be 
established in this way, but not in the way Tscherning explains it. 
On the other hand, the aberroscope, like Young’s optometer in this 
respect, is by no means sensitive enough to bring out the real com¬ 
plicated form of the caustic surface, because, on the whole, no con¬ 
clusion can be drawn from the curvature of the shadow-lines in the 
peripheral parts of the blur circle. Perhaps, it might be exceedingly 
well adapted for diagnosis of abnormal asymmetry, because it tells the 
sign of the transversal asymmetry, provided the sign of the curvature 
of the central shadow-line does not change when the image of the 
luminous point crosses from one side of the retina to the other. 
The change of sign of the aberration during accommodation as 
reported by Tscherning has not thus far been proved by investiga¬ 
tions with the aberroscope. Undoubtedly, as the writer himself has 
shown, the curvature of the shadow-lines can be seen to diminish 
during accommodation. But a change of this kind is bound to occur 
to some extent from the displacement of the image of the luminous 
point that is in front of the retina. It may be due partly also to the 
variation of the component of the distortion-value that is independent 
of the aberration-value. In order to show that the aberration varies 
during accommodation, it is absolutely necessary to prove first, by the 
method given above, that there is positive aberration in the case of 
emmetropic refraction (which means, of course, that the eye is properly


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