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 not 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.

What have we two eyes for? Why will not one eye do? Well, it is astonishing what people having but one eye are able to do with it, but that is a matter of great practice. Having two eyes, we are able to judge distances, and we judge the distance of an object partly by our knowledge of its real size, and partly by an involuntary estimation of the angle between the optic axes, when both eyes are directed towards the object; and we are able also, by means of our two eyes, to see bodies stereoscopically; to tell, for instance, whether their surface is concave or convex.

When I look at this bottle the image on the retina of my right eye includes more of the right hand side of the bottle, and that formed on the retina of my left eye includes more of the other side; so that between these images, which are different in the two eyes, I am able to understand that the bottle has a convex surface.

How is it that if the images are produced at the back of our eyes upside down, we see things as they are? There have been a lot of ingenious theories to explain this; one was that it had something to do with the junction of the optic nerves; but it is the simplest possible thing in the world, a pure matter of experience. We are accustomed, from our earliest infancy, to regard images that are produced on the lower part of the retina as coming from objects high up, and we are accustomed to regard images produced on the upper part of the retina as coming from bodies low down; and so, when we have an upside down image produced upon the retina, we see it the right way up, because the lower part corresponds, and the upper part corresponds with the whole experience of our lives.

The eyeball is moved in its cavity or orbit by four muscles called the recti, or straight muscles, which are attached to the sclerotic coat, and, passing backwards, are fixed to the bones at the hinder part of the orbit; a superior one above and an inferior one below, an external one outside and an internal one inside. When the upper one contracts it makes the eye look up, when the lower one contracts it makes the eye look down, when the external one contracts it makes the eye look outwards, and when the internal one contracts it makes the eye look towards the nose. There are two others; they start on the outer side of the eyeball, one above and one below. They are called the oblique muscles. One of these, the upper one (the superior oblique muscle) starts on the outer side, its tendon passing round a little notch in the bone, which forms a kind of pulley, and then it lies back along with the straight muscles. The inferior oblique muscle is attached below to the outer side of the eyeball, and by its other end to the floor of the orbit. When either of these contracts it pulls the eye round, so that between these six muscles the eye can be turned in all directions, and they are supplied by the nerves named in the last lecture.