Since the surfaces of the cornea, however, are practically parallel, we may neglect the difference between it and the aqueous humor, and look upon the two as one medium, having in front the shape of the anterior surface of the cornea, and behind, the anterior surface of the lens, so as to form a concavo-convex 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 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.
Fig. 224. Showing the course of the rays of light rom 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 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 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 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.
*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.
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.