the image of an object is formed nearly at the distance of the retina; and, moreover, the eye possesses the property of adjustment for different distances. If, therefore, the eye is adjusted to receive an impression of the object A B, then all the rays emanating from A will unite in a point a upon the retina, and those from B in the point b, and form upon the retina a small reversed picture a b. It is evident from the figure that all rays which do not pass through the optical centre are refracted so as to unite into an image upon the retina.1 When we look at a very distant object, a star for instance, the rays fall parallel upon the eye, and since they are united to a single point upon the retina, it follows in this case that the focus of the media of the eye falls exactly upon the retina. So perfect is, therefore, the formation of the eye, that the form of the refracting surfaces and the refracting power of the eye are exactly proportioned to the distance between the retina and the outer surface of the cornea. This is, of course, only the case with a normal eye, the relative proportions being different in a short-sighted or long-sighted eye. In this manner images of all kinds of objects are formed, upon the retina. the retina. The iris here plays an important part, since it increases the distinctness of the image. It serves as a kind of screen to the eye, In order to trace exactly the path of a ray in the eye, two optical centres must be found by calculation, a line must then be drawn from the luminous object to the first optical centre, and one parallel to it from the second centre to the retina. These two centres are situated very near to each other, and can practically be taken as a single centre. covering the edge of the lens and excluding the marginal rays, which, in every lens, render the picture indistinct, because their focus does not exactly coincide with that of the rays which pass through the central part of the lens. A similar screen is used in all optical instruments, in telescopes, miscroscopes, and in the camera obscura; otherwise the pictures are seen with indistinct edges. The iris, however, is a screen of a much more perfect description than those which are used in our instruments. It not only excludes the injurious marginal rays, but it also regulates the amount of light which is sufficient and necessary for the eye. When we direct the eye towards a luminous object, the sky or a flame, the pupil contracts, and enlarges considerably when we look in the dark. This contraction may be easily observed if a person is placed before a bright window with his eyes shaded by his hand, and the hand is suddenly removed. The pupils are considerably larger in the twilight than in broad daylight. In this manner the quantity of light which penetrates the eye is regulated, for the smaller the pupil the smaller is the quantity of light which penetrates it from any one point, and the larger the pupil the greater also will be the quantity of light. Night-birds, such as owls, are remarkable for their wide pupils, which enable them to see well in the dark, whilst they shun the daylight because they are blinded by it. If we return once more to our comparison of the eye with the camera obscura, we are struck with another advantage which it possesses over the latter instrument. This advantage lies in the fact that the background of the eye is a concave surface, whilst the surface of the ground glass plate which receives the image in the camera obscura is plane. The advantage to the eye is here twofold, which will immediately strike anyone who has examined a picture in the camera. The image which a convex lens produces of a large body with perpendicular lines, as, for instance, the arrow in fig. 8, is not perfectly perpendicular, but slightly curved, and this curvature may be exactly calculated by drawing a circle round the centre of the lens at the distance of the picture. Let us suppose the whole field of vision to be depicted by a lens, then the image formed by the lens is not thrown upon an even surface, but upon a concave surface, whose centre lies in the lens. This accounts for the lines towards the edges of the picture in a camera obscura never being perfectly perpendicular, but always more or less curved. This may be seen very plainly in photographs where the vertical side of a house is represented near the margin. This defect is not very great if the image lies near the optical axis (F F'), but as soon as the images are situated at too great a distance from it, they appear distorted and indistinct. This is the reason why the size of photographic pictures is limited, and that it is only possible to represent a small portion of the field of vision. It is different with the eye, since it possesses a concave background upon which the field of vision is depicted, and with which the curved form of the image coincides exactly. Thus the defect of the camera obscura is entirely avoided; for the eye is able to embrace a larger field of vision, the margins of which are depicted distinctly and without distortion. If the retina had a plane surface, like the ground glass plate in a camera, it must necessarily be much larger than is really the case if we were to see as much; moreover, the central portion of the field of vision alone would give a good clear picture. CHAPTER II. The Adjustment of the Eye-Short Sight and Long Sight. WE know from experience that we are able to see distinctly objects at different distances from our eye. But on careful observation we shall find that we cannot form simultaneously a perfect picture of objects at different distances from us. Suppose that we are in a room at a little distance from the window, and that we then hold up a finger a few inches before one eye, and close the other; if we now fix the eye steadily upon the finger so as to see it distinctly, the window in the background will be seen indistinctly. If we fix our eyes upon the window frame so as to see it distinctly, then the outline of the finger becomes indistinct, so that we can see distinctly at will either the window frame or the finger. Thus the eye is adjusted for objects at different distances, and this property of the eye is called its adjustment. When our eyes are wandering over objects at different distances from us, this adjustment is constantly at work, although we are quite unconscious of it ourselves. ab In fig. 8 the image of the arrow A B is depicted at |