Page 189 - Edited - Webster HEAD AND NECK - part 1

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us habitually work beyond the near-point (eg. the near-point may be at ~20 cms, but a book is held comfortably
on a lap comfortably on a lap or desk at ~30 cms or more from the eyes). For most people, therefore, the
progressive recession of the near point remains unremarked until it is so far away that in order to read (or sew,
or mend watches or whatever), the distance at which the object must be held in order to focus it is so great that
it is no longer possible to resolve the fine detail. This state (known as presbyopia) usually arrives during the
fifth decade of life (i.e. “the forties”), when reading spectacles are prescribed.

(iv) The pupil contributes to the refractive mechanism in as far as in bright light it is small and adds a
“pin-hole” diffractive effect to focusing, and also restricts use of the lens to its most central portion, so
minimising spherical aberration. Try experimenting with the smallest typeface you can read in varying light
intensities (from bright sunlight to the light from a failing torch, for example). You will find it possible to read
a much smaller typeface in bright light than in dim, i.e. visual acuity is dependent upon illumination, and
improves as the pupil constricts . During the act of accommodation three events occur in the eye: the ciliary
3
muscle contracts, the pupil constricts (note that in both cases - lens and iris - it is the parasympathetic nerves
which are involved, carried in the oculomotor nerves) and the eyes converge towards the nose (again, the
oculomotor nerve). These three events are known as the accommodation reflex. The pupil will, however,
quickly return to a size appropriate for the level of illumination once the object of interest is in focus.
(v) The physical parameters of the refractive mechanism are based, of course, upon a presupposed size
of the eyeball: in particular, great variations in the antero-posterior length of the eye will cause the image to be
focused behind or in front of the retina. In fact, such structural mismatches are the commonest causes of short-
and long-sightedness (myopia and hypermetropia) - at least before the fifth decade of life, when presbyopia
slips quietly on to the stage.
(vi) It is important to remember that the refractive mechanism is an inverting/reversing one, i.e. the
image of the outside world thrown on the back of the eye is upside-down and reversed left for right [Neuro
notes, Vol. II, Fig. 156). The mechanism can be simplified and considered as “the reduced eye” (Neuro notes,
Vol. II, Fig. 150) which treats the eye as a pin-hole device, with all rays passing through the node (i.e. the
imaginary pin hole). Because the node is a point located at a fixed distance (which turns out to be though this
is not important - 16.75 mm in front of the retina, 1.6 mm behind the cornea), the size of all objects and their
images can be expressed in angular terms. The angular size of an object is the same as that of its image
measured at the node, although their linear sizes may be vastly different. As a rough guide, remember that at
about 1 metre, an object of 20 mm linear size subtends 1° of arc (i.e. has an angular, at the node, of 1°) and
throws a retinal image ~0.25 mm long. This general approach is the basis of eye-testing charts (see below). (A
rule of thumb: at arm’s length, your thumb-nail subtends ~1° of arc.)
(vii) Finally, the image-forming mechanism is subject to chromatic as well as spherical aberration.
When light at the extreme red end of the spectrum is brought into focus at the retina, light at the violet end is
focused well in front. The possible contribution of the selective transmission properties of the lens to minimise
this problem has already been mentioned, i.e. the refractive mechanism selectively transmits so that the eye is
biased in favour or the red end of the spectrum.

K.E.W.

3 Note: Unquantified, this is a slightly unfair test, since the acuity of the retinal photoreceptors is also dependent upon illumination (see
Neuro Notes, Vol. I, p. 145) but this effect is “yes-no” rather than graded. Camera buffs will recognise this pupillary phenomenon
as “stopping down” which increases the depth of focus; i.e. the accommodation becomes less critical.

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