This section is from the book "Health", by W. H. Coefield.
I say that the place where the optic nerve enters the eyeball, where there are most nerve fibres, is blind, so we see that it is not nerve fibres that are affected by the stimulus of light; the layer of cones is, in fact, the part of the retina which is sensitive to light, and from it stimuli are conveyed along the nerve fibres from the retina to the brain, so that in the sense of sight, just as in the senses of taste and smell, the stimulus is given not directly to the nerve fibres at all, but to another structure altogether.
In the case of light, what movements are transferred in this way into stimuli in the optic nerve? Movements occurring in a very attenuated medium called ether, which permeates all space and all bodies. It is by movements of the particles of this ether that the stimulus is produced which gives us the sensation of sight.
The eye is an optical instrument, and it has certain arrangements which we find it well to imitate, as far as we can, in all our best optical instruments.
You will remember I told you that in the choroid membrane and in the iris, there are not merely bloodvessels and nerves, but layers of pigment. Now, what are those layers of paint for ? They have several uses; one of them is to absorb the rays of light which would, for one reason or another, interfere with the distinctness of the image that has to be produced on the retina.
Dark substances absorb light, and one of the reasons of the existence of colour in the eye is, that rays of light, which would otherwise be of no use, and not only be of no use, but be of harm, shall be absorbed. It is not the only use, but it is the only use I need mention here.
Why, you will say, is the iris placed in front of the crystalline lens ? Why are not all rays of light that fall upon the cornea allowed to go into the eye ? That curtain cuts off a great deal of the light that would otherwise fall on the crystalline lens, and pass into the eye. A distinct image of a body placed in front of the lens is made by rays that pass as nearly as possible through the centre of the lens, as the rays of light that pass from a body through the edges of a lens only serve to blur the effect; and so it is desirable, in constructing optical instruments, that means should be taken for ensuring the entrance of rays chiefly through the middle of the lenses; that is, then, the reason of the existence of this iris. In a telescope or microscope you will see at certain distances discs, with holes in their centre, called diaphragms; they prevent rays passing through the instrument otherwise than through the centre of the lenses; they are blackened, and the interior of the instrument is also blackened, in order to absorb all the rays of light, except those that pass through the middle of the lenses, and to prevent their being reflected from side to side, so as to cause confusion. The iris, then, is the diaphragm of the eye.
Now, I said that in this iris there were muscular fibres; these are of two kinds: there are muscular fibres that run round the aperture; circular fibres, that are supplied with nerves from the third pair of nerves, the nerves which supply most of the muscles connected with the eye; and there are fibres that run from these circular fibres towards the outside of the iris, radiating fibres, which axe supplied by branches from the great sympathetic system of nerves. When the circular fibres contract, they make the aperture smaller, and so shut out a certain quantity of light. When the radiating fibres contract they pull the circular band around the pupil outwards in all directions, because the iris is fixed round its edge, and so make the pupil larger. When do these contract? When too strong a light strikes upon the eye, and passes through the crystalline lens on to the retina, a reflex action occurs; the too strong light causes a great stimulus, which is transferred along the fibres of the optic nerve to the ganglia at the base of the brain; they then start another stimulus, quite independently of the will, which passes along the third pair of nerves, and causes that circular muscle round the pupil to contract, and make the pupil smaller, so that less light can penetrate into the eye.
When, on the other hand, you go into a dark room where the light is not sufficient for you to see, a command is sent along the sympathetic nerve, which is the nerve which stimulates most of the involuntary muscles of the body to act, to the radiating fibres for them to contract and enlarge the pupil, so as to admit more light into the eye The iris is thus a self-acting diaphragm; if there is too much light, it partially shuts up the aperture; if too little, it expands the pupil, so as to admit more.
Now, we are able to see objects at different distances quite clearly, and that is done by a self-acting adjustment, by means of which the shape of the lens, which is elastic, is altered, so that we can, if we look at an object close to us, have an image produced quite clearly upon the retina, and transferred from that to the brain; and the same is true if we look at a distant object.
Suppose that the image of an object, instead of being produced upon the retina, is produced in front of the retina, then an indistinct image of the object is produced upon the retina. The rays of light are in that case brought .to a focus too soon, or short of the retina, and the person is said to be short-sighted. Suppose, on the other hand, that the rays of light from the object would be brought to a focus, so as to produce a clear image behind the retina, they then form an indistinct image upon the retina, and you have what is called weak sight. In short-sighted people the rays of light are brought to a focus in front of the retina, and in weak-sighted persons (especially in old people) the focus is behind the retina, so that in the one case glasses are worn to prevent the rays of light coming to a focus too soon, and in the other to make them come to a focus quicker. Now, it is quite clear that a doubly convex glass would noi do the former, so a kind of lens is worn by short-sighted persons which is concave on both sides, and that glass has the property, instead of bringing the rays of light together, of diverging or separating them to a certain extent, so that they do not come to a focus so soon; and weak-sighted persons have to wear doubly convex glasses which will bring the rays of light together sooner.