Pretty nearly opposite the place where the white part of the outside coat of the eye joins the transparent part, there is fixed a transparent body, which we call the crystalline lens-shaped like what we ordinarily understand by a lens, a body which is convex on both sides. This divides the cavity of the eyeball into two- the front cavity between it and the cornea, and the posterior cavity between it and the retina. The front cavity is filled with a watery fluid, which goes by the name of the aqueous humour, and the posterior cavity with a gelatinous body, which goes by the name of the vitreous humour.

I will go on to describe the way in which this apparatus works, and I will explain a little more at length the structure of the sensitive part of it, viz., the retina.

When rays of light fall upon what we call a lens of glass, or of any transparent substance denser than air, which is convex on both sides, if they are parallel to one another like the rays of light that come from the sim, after they have passed through this transparent substance they are no longer parallel to one another, but are brought, for reasons I have not time to describe, into a point; that point is called the focus of the lens, and the distance of that point from the centre of the lens is called the focal length of the lens. When the rays of light proceeding from an object on one side of a convex lens, and passing through it to the other side of that convex lens, are received on a screen placed at a certain distance from it, these rays of light form an image of the object on the screen on the other side of that lens. The rays of light from the upper part of the object that pass through the centre of the lens, necessarily fall upon the lower part of the screen, and those which start from the lower part of the object, and pass through the centre of the lens, fall upon the upper part of the screen; as the rays which pass through the centre of the lens are most concerned in the production of the image on the screen, this is consequently upside down.

Now, in the eye we have a series of lenses. The transparent cornea is a lens, the aqueous humour is a lens, the crystalline lens is another, and the vitreous body is a lens; so that we have a series of lenses which we may call a compound lens, but, for the sake of simplicity, we must consider the action of the most important of them-the crystalline lens. From what I have said, you will see that the image of an object placed in front of the eye must be produced by this crystalline lens somewhere behind it, upside down, and if there is a curtain placed behind that crystalline lens, so that the image of the object in front can be produced upon that curtain, it will be produced there upside down. There is such a curtain, and that curtain is the inner coat of the eyeball, which we call the retina.

In the retina there are several structures, the most superficial consisting of the expansion of fibres of the optic nerve; there are other structures below that, and the deepest structure in the retina, next to the choroid membrane, is a layer, called, from the shape of the structures in it, the layer of rods and cones.

When, then, an external object is placed in front of the eye, its image is produced upside down upon the retina, distinctly if the latter is at a proper distance, if not, the image is still produced upon it, but indistinctly.

In this retina there are two spots, the structure of which differs from the structure of the rest of the retina. One of these is the place where the optic nerve goes in. The optic nerve does not enter the eyeball in the centre of the back of it, so to speak, or in what is called the axis of the eyeball, but at a point a little nearer to the nose. Now the point where that nerve enters is one of the peculiar spots in the retina, and the other is immediately opposite the axis of the eyeball.

When an image is produced upon the retina, a stimulus is caused which travels along the nerve fibres of the optic nerve to the brain, and produces the sensation of sight. Where is that stimulus started ?

The nerve fibres of the optic nerve are spread upon the surface of the retina, and is it the nerve fibres of the optic nerve that are sensitive to the light ? if so, the sense of sight differs in a remarkable manner from the three senses already described. Nothing of the kind; the spot in the retina which is most sensitive to the action of light is the spot called the yellow spot, which is in the axis of the eyeball In that yellow spot there are no nerve fibres at all, but on the other hand the layer of cones is highly developed. The most sensitive part of the retina, then, contains no nerve fibres at all I said that the spot at which the optic nerve enters is another peculiar spot: there you have nothing but nerve fibres, the other coats of the retina are not developed at all The nerve fibres of the optic nerve pass through the sclerotic and choroid coats, and then spread abroad on the inner surface of the rest of the retina, so that there is nothing but fibres where the optic nerve enters the retina; and the spot where the optic nerve enters is blind, so that the place where there are most nerve fibres is insensitive to light; you can prove it If you take a piece of paper, and make two marks upon it, three or four inches apart, and look with your right eye fixedly at the left-hand mark, placing the latter directly in front of the eye, shut the left eye, and then move your head towards and from the paper; in a little time you will find that in a certain position you can see the left-hand spot, but you cannot see the other; you will find that there is a certain position in which you can see one spot distinctly, but if you move your head either way, you can see both spots. Now, it has been shown by accurate experiment, that when that is the case, the image of the other spot is produced upon the place where the optic nerve enters the eyeball.