Eye disorders > The eye > Eye accomodation

Eye accomodation

In the normal eye, in a condition of complete repose, parallel rays come to a focus exactly on the rods and cones of the retina, and the
object from which the rays come is therefore seen distinctly.

Rays from a near object proceed in a divergent direction, and come to a focus behind the retina; the object would not then be clearly seen, unless the eye possessed within itself the power of bringing rays which are more or less divergent into union on the retina.

This power of altering the focus of the eye is called accommodation, and is due to an alteration in the form
of the lens. That the eye possesses this power can easily be proved in many-ways, apart from the conscious muscular effort; perhaps as simple a way as any to demonstrate it to oneself is to look through a net held a short distance off at sonne distant object.

Either the net or the object can be seen distinctly, but not both at once. If the meshes of the net be looked at, then the distant object becomes indistinct, and, on looking at the object, the meshes become confused.

Accommodation, therefore, increases the refraction of the eye, and adapts it to near objects. The changes which take place in the lens during accommodation are:

1. The anterior surface becomes more convex and approaches the cornea.
2. The posterior surface becomes slightly more convex, but remains equally distant from the cornea.

That these changes take place may be proved in the following manner :
a lighted candle, or other convenient object being beld on one side of the eye, so as to form an angle of 30� with its visual axis, an
observer looking into the eye from a corresponding position on the other side, will see three imaes of the flame ; the first upright, formed by the cornea, the second, larger, upright and formed by the anterior surface of the lens; the third smaller and inverted, formed by the posterior surface of the lens. When accommodation is put in force, images one and three remain unchanged in shape and position; image two, which is that formed by the anterior surface of the lens, becomes smaller, more distinct, and approaches image one, proving that this surface of the lens has become more convex and has approached the cornea.

In an emmetropic eye adapted for infinity, it has been proved that the radius of curvature of the anterior surface of the lens is 10 mm.; when accommodated for an object 13.5 cm, off, it is changed to
6 mm.

The pupil also becomes smaller, the central part of the iris advances, while the peripheral part slightly recedes.

The alteration in the slwhe of the lens is due to the contraction of the ciliary muscle, which draws forward the choroid, thereby relaxing the suspensory ligament and allowing the elasticity of the lens to come into play. This elasticity is due to the peculiar watch-spring arrangement of the lens fibres.

When the ciliary muscle is relaxed, the suspensory ligament returns to its former state of tension, and so tightens the anterior part of the capsule, flattening the front surface of the lens.

When the muscle is thus relaxed to its uttermost, the lens has assumed its least convexity, and the eye is then adapted far its far point (punctum rentotum) (r).

In this condition the eye is spoken of as being in a state of complete repose.
FIG. 29.

Diagram of lens, cornea, & c. The right balf is represented
as in a state of accommodation, the left half at rest.
A. The anterior chamber. C. The. cornea. L. The lens.
V. The vitreous humour. I. The iris. M. Ciliary muscle.

When the ciliary muscle has contracted as much as it can, the lens has assumed its greatest convexity, and its maximum amount of accommodation is now in force. This point is the nearest for which the eye can accommodate itself, and is called the punctum proximum(p).
In the emmetropic eye the punctum remotum is situated at infinity.

The position of the punctum proximum can be determined in several ways; the ordinary plan is to place in the patient's hand the small test-type, and note the shortest distance at which he can read No. 1
with each eye separately: or we may measure it with the wire optometer, which consists of a steel frame crossed by thin vertical wires; this is supported in a handle to which a tape measure is attached, the tape is placed against the temple, and held there while the frame is made gradually to recede from the patient's eye we are examining, stopping I as soon as the wires become distinct, and reading off the number of centimetres on the measure.

Another excellent plan by which to find the position of the punctum proximum is that of Scheiner; close in front of the eye we wish to examine is placed a card pierced with two small pin-holes, which must not be further apart than the diameter of the pupil.

Through these two holes tho patient is directed to look at a pin held about one meter away (the other eye is of course excluded from vision during the experiment); the pin will be clearly and distinctly seen, we then gradually approach it to the eye; at a certain place it will become double, the point at which the pin ceases to appear single will be the punctum proximum.

In Fig. 30, the biconvex lens L represents the eye, D the perforated card, P the pin, E E' the two sets of rays from P, which focus exactly at B, the, retina. If, however, the pin be brought nearer, so that the accomodation is unable to focus the two sets of rays, they will form, instead of one, two images of the pin on the retina as at A. These will be projected outwards as crossed images.

The distance between the punctum remotum and the punctum proximum is called the range of accommodation.

The force necessary to change the eye from its punctum remotum to its punctum proximum is styled the amplitude of accommodation. The amplitude of accommodation, therefore, is equal to the difference between the refracting power of the eye when in a state of complete repose, and when its maximum amount of accommodation is in force, and may be expressed by the formula:
a = p - r

A convex glass placed in front of the eye produces the same effect as accommodation, i.e. it increases its refraction and adapts the eye for nearer objects. We can easily understand that the lens which enables an eye to see at its near point without its accommodation,
is a measure of the amplitude of accommodation, giving to rays which come from the near point a direction as if they came from the far point.

The amplitude of accommodation is much the same in every kind of ref raction. If we wish to measure it in an emmetrope, we have merely to find the nearest point at which the patient can read small print. A lens whose focal distance corresponds to this is a measure of the amplitude of accommodation. Thus, supposing 20 cm, the nearest distance at which the patient is able to read small print, we divide this into 100 cm to find the focal distance of the lens (100/20 = 5 D.) and in this case a lens of 5 D. is the measure.
we require.

Or we can measure it in an inverse manner by looking at a distant object through a concave glass; the strongest with which we can see this distant object distinctly is the amplitude of accommodation, the concave glass giving to parallel rays coming from the distant object such an amount of divergence as if they came from a point situated at the principal focal distance of this glass.

Therefore the amplitude of accommodation in emmetropia is equal to the refraction when adapted to its punctum proximum, and may be expressed by the formula:

a = p - ∞ or
a = p - 0 or
a = p

The Accommodation of Hypermctropes - A hypermetrope requires some of his accommodation for distant objects ; we must, therefore, in order to find the amplitude of accommodatiom in his case, add on to the lens whose focal length equals the distance of the near point, that convex lens which enables him to see distant objects without his accommodation, and we express it by the formula

a = p + r

Thus, to take an example, we will assume the patient's near point to be 25 cm. (100/25 = 4 D.) and that he has to use 4 D. of accomodation for distant object; then the amplitude of his accommodation would be 4 D + 4 D = 8 D.

The Accommodation of Myopes - In a myope we have to subtract the glass which enables him to see clearly distant objects, from that whose focal length equals the distance of the near point. The formula will then be

a = p - r

Thus, to find the amplitude of accommodation in a myope of 2 D., the near point being at 10 cm, we subtract from (100/10=10) 10 D, the amount of the myopia, 2 D, and the resulting 8 D. is therefore the amplitude of accommodation.

Hence it is obvious, that with the same amplitude of accommmodation, the near point is further away in hypermetropia than in emmetropia, and further in emmetropia than in myopia. Thus an emmetrope,
with an amplitude of accommodation of 5 D., would have his near point at (100/5=20) 20 cm.; a hypermetrope of 2 D., whose amplitude equalled 5 D., would require to use 2 D, of his accommodation for distance, leaving him 3 D., which would bring his near point to
(100/3=33) 33 cm.; and a myope of 2 D., who would require a concave glass of this strength to enable him to see at a distance, would have a near point of 14 cm. (100/7=14) with the same amplitude.

Accommodation is spoken of as absolute, binocular, and relative.

Absolute is the amount of accomodation which one eye can exert when the other is excluded from vision.

Binocular, that which the two eyes can exert together, being allowed at the same time to converge.

Relative, that which the two eyes can exert together for any given convergence of the visual lines.

Fig. 31 diagramatically represents the amplitude of accomodation in emmetropia.
As age advances the elasticity of the lens diminishes, the accomodation therefore becomes less, and the near point gradually recedes. These changes commence at a very early age, long before the individual has come to maturity.

The following table gives the amplitude of accomodation at different ages.

Years Amplitude of accommodation
10 14 D.
15 12 D.
20 10 D.
30 7 D.
40 4.5 D.
50 2.5 D.
60 1 D.
75 0