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 stinmlus 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 of the existence of colour in the light, which would otherwise be of be of no use, but be of harm, shall not the only use, but it is the only here.

Why, you will say, is the iris pl crystalline lens? Why are not all fall upon the cornea allowed to go i curtain cuts off a great deal of otherwise fall on the crystalline len eye. A distinct image of a body lens is made by rays that pass through the centre of the lens, as pass from a body through the edges to blur the effect; and so it is desir optical instruments, that means a ensuring the entrance of rays chiefly of the lenses; that is, then, the res of this iris. In a telescope or mic at certain distances discs, with h called diaphragms; they prevent the instrument otherwise than thro lenses; they are blackened, and instrument is also blackened, in ore rays of light, except those that pass of the lenses, and to prevent their side to side, so as to cause confusion the diaphragm of the eye.

Now, I said that in this iris 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 are 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.