in front the shape of the anterior surface of the cornea, and behind, the anterior surface of the lens, so as to form a concavoconvex lens. We thus have only three media to consider, viz., (1) the aqueous humor and cornea; (2) the lens and its capsule; and (3) the vitreous humor. And only three refracting surfaces need be enumerated, viz., (1) the anterior surface of the cornea; (2) the anterior surface of the lens; and (3) the posterior surface of the lens.

These refracting surfaces may all be looked upon as portions of spheres whose centres lie in the same right line, and hence may be said to have a common axis. The eye may be regarded as an optical system, centred round an axis which passes through

Showing the course of the rays of light from two luminous points to the retina. The rays from the point a on passing through the cornea, lens, etc., are collected on the retina at b. Those from a' meet at b', and thus the lower point becomes the upper.

the middle point of the cornea in front, and the central depression (fovea centralis) of the retina behind. This is spoken of as the optic axis of the eye.

The rays of light entering the eye are most strongly refracted at the surface of the cornea, because they have to pass from the rare medium, the air, to the denser cornea and aqueous humor. So also more bending of the rays occurs between the aqueous humor and the anterior surface of the lens than between the posterior surface of the lens and the vitreous humor.

The lens is not of the same density throughout, but denser in the centre, and being made up of layers, the central part refracts more than the outer layers.

The manner in which the inversion of the image is produced by a convex lens is shown in the preceding figure, in which the lines correspond to the rays passing from two points through the lens." If the arrow a a' be taken for the object, from either extremity of it rays pass through, and are more or less bent by the lens. It will be sufficient to follow the course of three rays from the head of the arrow. One of these passes through the centre of the lens, and leaves it in the same direction which it entered, because the two surfaces at the points where it entered and left may be regarded as parallel, and so cause no refraction. The rays which do not pass through the centre are bent on entering and on leaving the lens, so that they all meet at the same point and there produce an image of the head of the arrow, at b'. In the same way the feather end of the arrow is produced at b; the position of the image of the object is thus reversed by the light rays passing through the lens.

In a biconvex lens, with two surfaces of the same degree of convexity, the central point through which the rays pass without being refracted is easily made out, as it is the geometrical centre of the lens. This central point is spoken of as the optical centre. With systems of lenses of varying convexity, and of more than one in number, as we have in the eye, where the rays of light are bent at different surfaces, it is much more difficult to determine the optical centre. However, by means of the measurements made by Listing, two points close together are known, which may be said to correspond practically with the optical centres of the eye; they lie in the lens, between its centre and posterior surface. The path of the various rays may thus be exactly made out.*

The rays which come from a distant luminous point and fall upon the eye, are refracted by the cornea and aqueous humor, so as to be made convergent on their way to the lens; they are then

* The impossibility of making clear the important relationships, such as nodal points, and other constants of the eye in a short text-book, and the deterrent effect exerted upon the mind of a junior student by brief incomprehensible statements, have induced the author to omit this part of the subject. He must refer those who are anxious to learn the cardinal points of the eye, to the more advanced text-books.

further bent at the surfaces of the lens, so that they are brought exactly to a point on the retina. That is to say, for distant luminous points, the retina lies exactly in the plane of focus of the dioptric media of the normal eye.

This convergence of the rays to a point on the retina, is the first essential for seeing clear and distinct images; for if the rays from each point of a luminous body were not united on the retina as points, the effects of the different rays from the various points of a body would become mixed, and there would be loss of definition of its image.

The rays from any bright point which enter the eye through the pupil may be imagined to form a luminous cone, the point of which lies at the retina, and its base at the pupil. After their union at the point of the cone, the rays would diverge again if the retina were not there to receive them.

SCHEINER'S EXPERIMENT.

It may be seen from the foregoing figure that if the retina, which normally would lie at 2, were placed nearer the dioptric apparatus, say at 1, or further from it, at 3, it would not meet the exact point of the luminous cone, but would receive the rays either before they came to a point, or after they had diverged from it. Thus indistinct rings of light would be seen instead of one luminous point, and an image would be blurred and indefinite.

From this it follows that the eye, when quite passive, can only get an exact image of bodies which are placed at a certain distance from it, just as, for any given state of a camera, only those bodies in one plane come into focus and give a clear picture on the screen. If the dioptric apparatus of the eye were rigid and unalterable, since the relation of the retina to it is permanently the same, we could only see those objects clearly which are at a given distance from the eye. We know, however, that we see as distinct an image of distant as of near objects, and we can look through the window at a distant tree, or can adjust our eyes so as to see a fly walking on the window pane. We cannot see both distinctly at the same moment. This power of focusing may

be demonstrated by what is known as Scheiner's experiment, which is carried out in the following way.

FIG. 225.

270

To illustrate Scheiner's experiment; for explanation, see text.

Two pin holes are made in a card at a distance from each other not wider than the diameter of the pupil. The card is then brought close to the eye, so that a small object-such as the head of a bright pincan be seen through the holes. The dioptric media being fixed, moving the object nearer to or further from the eye would have the same effect as changing the relation of the retina to m n or p q in Fig. 225, by means of which we may explain the following observations: (1) The eye being fixed upon the object (of which only one image is seen), move the pin rapidly away; two objects now appear, showing that the rays coming through the holes have met before they reach the retina as at p q. (2) Move the pin near the eye; again two very blurred objects are seen, for the rays have not met when they strike the retina, as at m n. (3) Keeping the object in the same position, alter the gaze, as if to look first at distant and then at near objects; in both extremes two images are seen. (4) When the object is in exact focus, as at c, the closure of one of the holes does not affect the single image. (5) When two images are seen, closing the right-hand hole at g causes the right or left image to disappear, according as the focus c falls short of m n or is beyond p q, the retina. (6) By moving the pin's head nearer the eye, a point is reached at which the object cannot be brought to a focus as a single image. This limit of near accommodation marks the near point. A little attention teaches us that looking at the near object requires an effort which looking at the distant one does not; in fact, we have to do something to see things near us distinctly. This act is the voluntary adjustment of the eye which we call its accommodation for near vision.

ACCOMMODATION.

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 without 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