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/lit39649/434/
410 
Dioptries of the Eye 
[349. G. 
Since the contraction of the ciliary muscle is accompanied by a 
change of form of the lens merely until, in consequence of the shifting 
of the parts of the lens, this change reaches its maximum extent, where¬ 
as the muscle itself, as we know from the pupillary contraction and 
the trembling that occurs in the end, may be contracted beyond this 
point; therefore only a part of the contraction of the ciliary muscle is 
manifest, the rest is latent. According to Hess, the transition begins 
when the actual near point is reached, whereas, during the latent 
contraction of the ciliary muscle, there is merely a small apparent 
displacement of the near point further towards the eye, which depends 
on the concomitant contraction of the pupil.1 Consequently, an 
accommodation of two dioptries, at an age when this is all the power of 
accommodation available, does not make any higher demands on the 
ciliary muscle than the same amount of accommodation at a younger 
age. This is a very important fact for the symptomatology of pres¬ 
byopia; but in connection with it the contraction of the pupil due to 
latent contraction of the ciliary muscle, especially if the habitual 
size of the pupil is not very small, is another factor to be taken into 
account, because this by itself often decoys an uncorrected presbyope 
into trying to accommodate beyond his strength. Hess called atten¬ 
tion to another important consequence of the mechanism of accom¬ 
modation, namely, that we have the right to conclude from a normal 
amplitude of accommodation that the action of the ciliary muscle is 
normal also, because a paresis of the ciliary muscle does not become 
apparent until the restriction of movement extends to the region of 
manifest contraction. So far as physiological optics is concerned, this 
fact is significant as showing, as was remarked before, that Tschern- 
ing’s opinion, as to the performance of the ciliary muscle not being 
impaired by cocaine, cannot be proved by its effect on the amplitude of 
accommodation, and therefore is without any basis at present. 
There has been quite a lot of discussion in ophthalmological 
literature of an astigmatic accommodation. Without going into this 
subject in detail, it may simply be stated that no known facts indicate 
the possibility of a voluntary change of astigmatism by accommoda¬ 
tion or by the practice of astigmatic accommodation. However, it 
may be possible that the normal inverse lenticular astigmatism or the 
lenticular astigmatism that is present in the higher degrees of ocular 
1 * The physical or manifest near point (what is usually meant by the “near point of 
the eye”) is the point that is conjugate to the point where the optical axis meets the retina 
when the surfaces of the crystalline lens are most curved, the zonule being relaxed. The 
physiological or latent near point (whose position cannot be ascertained by any' method at 
present available) is the point on the optical axis for which the eye would be focused when 
the ciliary muscle is contracted to its utmost power, supposing there w'ere no limit to the 
effort of the crystalline lens to become spherical. (J. P. C. S.)
        

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