eyeball which is left uncovered by the open eyelids, the part of the light that falls upon the white of the eye is irregularly reflected, and renders visible that part of the eyeball. Those rays of light which fall upon the cornea pass through it. The exterior rays fall upon the iris, by which they are irregularly scattered, or reflected, and render it visible. The internal rays pass through the pupil, and are incident upon the crystalline, which, being transparent, is also penetrated by them, from which they pass through the vitreous humour, and finally reach the retina, upon which they produce an illuminated spot.

On the Law of Visible Direction.—When a ray of light falls upon the retina, and enables us to see the point of an object from which it proceeds, it becomes an interesting question to determine in what direction the object will be seen, reckoning from the point where it falls upon the retina. Let F be a point of the retina on which the image of a point of a distant object is formed by means of the crystalline lens, supposed to be LL. Now, the rays which form the image of the point at F fall upon the retina in all possible directions from LF to LF, and we know that the point F is seen in the direction of EC R. In the same manner, the points ƒƒ are seen somewhere in the directions ƒ's, ƒT. These lines FR, ƒ's, fr—which may be called the lines of visible direction-may either be those which pass through the centre, c, of the lens L L, or, in the case of the eye, through the centre of a lens equivalent to all the refractions employed in producing the image; or it may be the resultant of all the directions within the angles L F L, LfL; or it may be a line perpendicular to the retina at F, ff. In order to determine this point, let us look over the top of a card at the point of the

eyeball consists of four coats or membranes-namely, the sclerotic coat, the choroid coat, the cornea, and the retina; and these coats enclose three transparent fluids or humours—the aqueous humour, the vitreous humour, and the crystalline humour, or, as it is sometimes called, the crystalline lens.

The Sclerotic Coat, a a a, or the outside coat, upon which the maintenance of the form of the eye chiefly depends, is a strong, opaque, tough membrane, composed of bundles of strong white fibres, interlacing each other in all directions. This membrane covers about four-fifths of the external surface of the eyeball; leaving, however, two circular openings a large one, in front, which is covered by a transparent concavoconvex piece of nearly uniform thickness, called the cornea; and a smaller one, behind, at the entrance of a nerve called the optic nerve, which, proceeding backwards and upwards, and passing through holes in the skull, terminates in the brain. It is by this nerve that the impressions made by external objects on the organ of vision are conveyed to the brain. The muscles which give motion to the eyeball are attached to the sclerotic coat, which constitutes the white of the eye.

The Choroid Coat is a delicate membrane, lining the inner surface of the sclerotic, and covered on its inner surface, generally, with a black substance. In some people this substance is white, in consequence of which they are enabled to see pretty well in the dark, but imperfectly in the light.

The Cornea, bb (Fig. 50), is an exceedingly tough membrane. It is closely united at its edge with the corresponding edge of the sclerotica. It is slightly elliptical in its form, its horizontal being rather longer than its vertical diameter. Its external surface is more convex

than that of the sclerotica, so that it forms a segment of a sphere, smaller than that of the general surface of the eyeball. It therefore projects outwards in front of the eye, making the axis of the eye, which passes through its centre, a little longer than the diameter, which is at right angles to it, or cuts it square across the middle. The cornea being of nearly uniform thickness, the concavity of its inner surface corresponds with the convexity of its outer, and gives the whole the form of a common watch-glass, or a concavo-convex lens whose surfaces have equal radii.

The Retina. Within the choroid, and close to its black substance, lies the retina, rrrr (Fig. 50), which is the innermost coat of all. It is a delicate, reticulated, and perfectly transparent membrane, formed by the expansion of the optic nerve over the chief part of the internal surface of the eyeball. It is spread over nearly all the back and side parts of the surface, and terminates near the margin of the frontal opening covered by the cornea, already described. At the extremity of the axis of the eye, in a line passing through the centre of the cornea, and perpendicular to its surface, there is a small hole with a yellow margin, called the foramen centrale, which, notwithstanding its name, is not a real opening, but only a transparent spot, free of the soft pulpy matter of which the retina is composed.

Iris.—In looking through the cornea from without, we perceive a flat, circular membrane,'cc (Fig. 49), within, called the iris, which is grey, hazel, blue, or black, and divides the fore part of the eye-or that part between the crystalline lens and the cornea-into two very unequal parts. In the centre of it there is a circular opening, called the pupil, which expands when a small portion of light enters the eye, and contracts

object whose image is at F, till the edge of the card is just about to hide it; or—what is the same thing-let us obstruct all the rays that pass through the pupil, excepting the uppermost RL; we shall then find that the point whose image is at F is seen in the same

B

R

R

Fig. 51.

direction as when it was seen by all the rays LF, CF, LF. If we look beneath the card in a similar manner, so as to see the object by the lowermost ray RL F, We. shall see it in the same direction. Hence it is manifest that the line of visible direction does not depend on the direction of the ray, but is always perpendicular to the retina. This important truth in the physiology of vision may be proved in another way. If we look at the sun over the top of a card, as before, so as to impress the eye with a permanent spectrum by means of rays, LF, falling obliquely on the retina, this spectrum will be seen along the axis of vision, F c. In like manner, if we press the eyeballs at any part where the retina is, we shall see the luminous impression which is produced in a direction perpendicular to the point of pressure; and if we make the pressure with the head of a pin, so as to press either obliquely or perpendicularly,

we shall find that the luminous spot has the same direction.

Now, as the interior eyeball is as nearly as possible a perfect sphere, lines perpendicular to the surface of the retina must all pass through one single pointnamely, the centre of its spherical surface. This one point may be called the centre of visible direction, because every point of a visible object will be seen in the direction of a line drawn from this centre to the visible point. When we move the eyeball, by means of its own muscles, through its whole range of 120°, every point of an object within the area of the visible field, either of distinct or indistinct vision, remains absolutely fixed; and this arises from the immobility of the centre of visible direction, and, consequently, of the lines of visible direction joining that centre and every point in the visible field. Had the centre of visible direction been out of the centre of the eyeball, this perfect stability of vision could not have existed. If we press the eye with the finger, we alter the spherical form of the surface of the retina; we consequently alter the direction of lines perpendicular to it, and also the centre where these lines meet; so that the directions of visible objects should be changed by pressure, as we find them to be.

Erect Vision from an Inverted Image. As the refractions which take place at the surface of the cornea, and at the surfaces of the crystalline lens, act exactly like those in a convex lens, in forming behind it an inverted image of an object—and as we know, from direct experiment, that an inverted image is formed on the retina-it had long been a problem how an inverted image produces an erect object. The law of visible direction, above explained and deduced from direct