The difference of distance for which we can adjust our eyes is great, so that our range of distinct vision is very extensive. As already stated, the normal eye is considered to be constructed so that parallel rays of light, i. e., those coming from practically infinite distance, are brought to a focus on the retina. This is why we see the stars - which are practically infinitely remote from us - as mere luminous points. It is, therefore, impossible to fix a far limit to our power of distant vision. The nearer an object is brought to our eyes, the more effort is required to see it distinctly, until at last a point is reached where we cannot get a clear outline, no matter how we "strain our eyes." For a normal eye, called the emmetropic eye, this near limit is about 12 cm. or 5 inches, but it varies in different individuals.
For objects that are over 10 metres distance, very little change in the eye is required to see each distinctly, and the nearer the object approaches, the more frequently the adjustment of the eye has to be altered to see it clearly. When the eye is focused for any point within the limits of distinct vision, a certain range of objects at different distances from the eye can be recognized wuthout moving the adjustment. The range of this power is measured on the line of vision, and called the focal depth. In the distance we can take in a great depth of landscape, without effort or fatigue; but when looking at near objects the focal depth is less, and we must constantly accommodate our eyes afresh in order to see clearly objects at slightly different distances because of the shallowness of the focal depth in the nearer parts of visual distance.
The method by which the accommodation of the eye is effected differs from anything that can be applied to an artificial optical instrument, and is more perfect.
The following alterations are observed to occur in the eye during active accommodation, i. e., when looking at near objects: (1) The iris contracts so that the pupil becomes smaller; (2) the central part of the anterior surface of the crystalline lens moves slightly forward, pushing before it the pupillary margin of the iris, so that the lens becomes more convex; (3) the posterior surface of the lens also becomes more convex, owing to the general change of shape of the lens, but the centre of this surface does not change its position; (4) both eyes converge.
These changes can be seen in the accompanying diagram, showing a section of the lens, cornea and ciliary region (Fig. 226), in the left-hand side of which the lens is drawn in the position it assumes when accommodated for near objects. These movements can be seen in life by observing the changes in relative positions, etc., of the reflections of a candle flame thrown from the cornea and the two surfaces of the lens. On the cornea is seen a bright upright flame: next comes a large diffused reflection from the anterior surface of the lens, and at the other side of this a small, inverted image of the flame reflected from the posterior surface of the lens. When the adjustment is changed by looking from a far to a near object, the image on the front of the lens becomes smaller and moves toward the centre of the pupil. The image on the back of the lens also becomes smaller, but does not change its position. The amount of movement has been accurately measured by a special instrument called an ophthalmometer. The motions can be more exactly studied by means of the phakoscope, a dark box, in which prisms are placed before the observed eye, and each image is made double. The change in relative position of the two is more readily recognized than a mere change of size of the one.
Fig. 226. Diagram showing the changes in the lens during accommodation. The muscle on the right is supposed to be passive as in looking at distant objects, the ligament (L), is, therefore, tight, and compresses the anterior surface of the lens (A) so as to flatten it. On the left the ciliary muscle (M) is contracting so as to relax-the ligament, which allows the lens to become more convex. This contraction occurs when looking at near objects.