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.)