so dazzle it, besides considerably injuring the distinctness of the image in the eye. The iris is also provided with black pigment similar to that of the choroid, and assumes a lighter or darker colour, according to the number of these pigment-cells. In a blue eye the iris has only a thin layer of pigment-cells on its posterior surface, and has, therefore, a bluish appearance from the outside. The greater the number of pigment-cells the darker is the colour of the eye; this variation in the amount of pigment is the cause of the different shades which we see in eyes, from the darkest brown to the lightest blue or grey. In all races of men individuals may be found in whom this pigment is wanting, not only in the eye, but throughout the body, in the hair and skin; such people are called Albinos. The want of this pigment in the eye injures the sight in a remarkable manner. The iris has a whitish-red appearance, the pupil is generally bright red, and the dazzling effect of the daylight causes a winking of the eyelids, which is an endeavour to replace the protection of the pigment which is wanting. We also find albinos among animals: for example, the white rabbit, upon whose eyes many interesting observations may be made. Thus, for instance, a delicate reversed picture of the surrounding objects may be seen upon the posterior surface of an eye freshly extracted from such an animal, shining through the transparent coat of the eye, whilst this is not the case with the dark-coloured eye of another animal.

Now within the choroid is situated a delicate membrane called the retina (fig. 7, i). It forms the continuation and extension of the optic nerve, which, like the stalk of an apple, penetrates the posterior side of the

eye in a somewhat slanting direction with the side of the nose, pierces the sclerotic coat and choroid, and then spreads out on all sides, so as to form, by means of the peculiar terminations with which it is provided, a kind of nerve-carpet, which is the most delicate sensory organ created by nature. The retina, towards the front of the eye, touches with its edge the outer circumference of the iris, and lies quite open to the transparent interior of the eye, so that the rays of light fall directly upon it, and create in it an impression of light. Before, however, the rays of light reach the retina, they pass through a number of transparent organisations which are situated in the circular hollow of the eye, and are here ingeniously fitted together like the parts of a microscope or telescope, with this difference only, that they are packed tightly together, so as to allow no trace of air between them. The outer covering of this transparent body is the cornea, mentioned above; then, towards the interior, follows first the aqueous humour, w, secondly the crystalline lens, L; and thirdly, the vitreous humour, G. The aqueous humour fills, as we have seen, the space between the cornea, the iris and the lens. Directly behind the iris lies the crystalline lens, L, so well known, and resembling a very thick burning-glass. It is more convex upon its posterior than upon its anterior surface; in a living eye it is as clear as crystal, and consists of a somewhat soft substance, which becomes harder towards the back of the lens. It has been discovered that this substance is not the same throughout, but that it consists of small vessels which are arranged in intricate lines, thus giving a sexradiated structure to the lens. The lens, however, does not lie unconfined behind the iris, but is enclosed in a

transparent capsule, which, again, is held in position by a peculiar elastic membrane, k k and e e, of which more hereafter. It is sufficient to say here that this membrane joins the circular hyaloid membrane, which lies directly upon the retina. The spheroidal space between the lens and the retina is filled by the vitreous humour, a clear gelatinous mass, which is directly surrounded by the hyaloid membrane.

The rays of light, therefore, which fall upon the eye penetrate the cornea, the aqueous humour, the crystalline lens, and the vitreous humour, before they reach the retina, and on their way are refracted in such a manner that they unite into a distinct picture upon the background of the eye.

It is well known that with the aid of a glass lens we can throw a representation of any object upon a screen. If, for example, we take the front convex lens out of a pair of opera-glasses, and, holding it opposite a window, place a piece of paper behind it to act as a screen, a small reversed image of the window will appear upon it, which, upon holding the paper at a certain distance, will become clear and distinct. Again, the camera obscura is well known, which in its simplest form consists of a box, in one side of which a convex lens is fixed in an opening, while the opposite side consists of translucent paper, or a piece of ground glass on which the picture is formed. In principle the action of the eye resembles that of a camera obscura, which is commonly used in the production of photographic pictures; but the eye, as we shall see, is in many respects much more perfect than the camera obscura.

The manner in which a convex lens is able to

produce an image of an object is seen in fig. 8. Let the arrow A B represent an object at a certain distance from the lens, F and F represent the foci of the lens, the distance of which is easily found by allowing a sunbeam to fall upon the lens, and finding the point of convergence upon a screen, which with a strong glass is very bright and hot. In the focus all the rays unite. which are parallel to each other, and fall perpendicularly upon the lens; and, on the other hand, when rays of light fall upon the lens from the focus they take a parallel direction on the other side. The upper point B

of the object, amongst others, sends a ray upon the lens. which passes through the centre O. This ray suffers no refraction, because the lens stands in the same relation to it as if it were a pane of glass bounded by parallel surfaces. A second ray passes through the focus F, strikes the lens, and is continued on the other side in a line parallel to the line F' O F, which is called the optical axis. Both rays meet in the point b, and here, therefore, an image of the point of the arrow is formed. The image of the head of the arrow a is formed in exactly the same manner, as may be seen from the figure, and thus a reversed image of the object is formed, which decreases in size the further the object

is removed, and which, when the object stands at an immense distance, as, for instance, the sun, contracts to a single point-the focus.

Fig. 9 shows a camera obscura as it is used by photographers. In the brass tube hi is the lens; g is the ground glass plate upon which the image is to be received, and the case a b consists of two parts, which slide in and out, and enable us to find the position of the image for objects at different distances. Besides this, there is a screw on the brass tube which allows

the lens to be moved backwards and forwards, so that the picture may be sharply defined upon the glass plate. We shall see that the eye also possesses an apparatus which serves the purpose of forming distinct images, but which acts in a very different manner.

The refraction of the rays of light in the eye is much more complicated than in a simple convex lens; for, in the eye, the ray of light passes through several substances, and is refracted at the surface of each substance. The common result, however, produced by all these refracting media of the eye is exactly the