VISION. 176 VISION. ing. Rays from difl'eieiit points of the object must be focused at conespomling points on the retina, and every radiant ])oint of the object must have a corresponding focal point in the image (Fig. 1). If, for instance, the usually transparent Fig. 1. Illustrating the formation of an image of an external object upon the retina. cornea be rendered opaque by some disease, the person ■will still perceive light, but will be un- able to recognize objects. The e3'e serves as an optical instrument which leceives and focuses upon the retina rays of light from an object; it is often likened to the photographic camera, an interesting comparison that is founded upon many points of resemljlance. Thus, the camera incloses a dark chamber, the walls of which are blackened so as to prevent reliection and the resulting irregular illumination which would interfere with the definition of images; in front, it presents an opening for the admission of rays of light; in this aperture is placed a glass lens or series of lenses designed to refract the rays of light and bring them to a focus upon the sensitive plate placed in the back of the instrument ; the latter is capable of being moved forward or backward so that its position will correspond to the focus of the rays. Compared with the camera, the eye possesses all these parts, arranged, of course, in a much more delicate and wonderful manner. It is a dark chamber, lined with the black pigment of the retina and choroid, presenting an ojjoning in front for the admission of light (the pupil), and ])rovided with a sensitive layer (the retina). The focusing is accomplished by the transparent media of the eyeball (cornea, aqueous humor, lens, and vitreous humor), of which the lens plays the most important part. The marginal rays, which would cause a blurring of the image (spherical aberration) and a chromatic ring (chromatic aberration), and which are cut off in the cainera by a series of stops or diaphragms, are excluded in the eye by a colored curtain, the iris. The opening in the iris, the pu])il, en- larges or diminishes in size according to various conditions presented. In a bright light and when the eye is focused upon near olijccts, the pupil contracts; in a dim light and with distant vision, the pupil enlarges. These changes occur uncon- sciously and automatically, whereas in the camera the adjustment is effected by mechanical means .nnd requires different (lia|)hr:igms for various changes in the distance of the object and the intensity of illumination. AccoMMOHATioN. The focusing apparatus of the eye is of wonderful construction. Yhen we look at a distant object, the lens is kept Hat by the tension of its capsule and its suspensory ligament, and is conseqiiently less refractive; rays proceeding from a distant object being less divergent than from a near source, less power is required to focus such rays upon the retina. When the eye is fixed upon a near object, the rays coming from the latter being more divergent, greater refractive power is necessarv in order to produce a distinct image upon the retina. Uiuler such circumstances, the lens increases its convexity, especially that of its anterior sur- face, through relaxation of its ligament and cap- sule, and consequently increases its power. This change in the thickness and power of the lens, adapting the focus of the eye to various distances, is known as accommodation. It is due to the action of the ciliary muscle, which bj' its contrac- tion relaxes the capsule of the len.s, and thus allows the latter to bulge anteriorly. The fact that muscular action is responsible for this in- crease in the convexity and hence in the refrac- tive power of the lens explains why the con- tinued use of the eye for near vision results in fatigue. We can look at distant olijects, such as scenery, for long periods with a feeling of com- parative rest ; but when reading a book or en- gaging in other similar use of the eyes, we focus for near objects: and the continued action of the ciliary muscle under such circumstances causes fatigue, which is just as natural as the tired sensation of the legs following long continued action of the muscles of these extremities in walking. Acconnnodation in the adult has a range from infinite distance to about six inches. After the age of forty-five the human lens be- comes less elastic; as a result there is gradual loss of power to adjust the eye for near objects (presbyopia or old sight), which requires for its correction the use of spectacles containing con- vex lenses. The Retik.. This is the portion of the eye sensitive to light waves, and is described under Eye. The particular part thus sensitive is the layer which is called the rods and cones. This layer is composed of great numbers of jninute rod-shaped and cone-shaped bodies. Light en- tering the eye influences the rods and cones in some manner not yet understood, though the changes are supposed to be photochemical in their nature; these changes in the rods and cones, produced by the vibrations of the luminiferous ether, result in stimuli which are transferred to the fibres of the optic nerve. Stinuilation of the retina, in whatever way produced, is accompanied by the sensatioM of light; thus, with pressure on the eyeball there are luminous impressions ( phosphenes) , and with electrical stimulation, irritation of any sort, or division of the 0]itic nerve there is the sensation of flashes of light. In the outer jiortion of the rods, thoiigh not in the cones, is a purplish-red matter known as the visual piirple. or the rod-pigment; this bleaches upon exposure to light and is regen- erated during darkness or a faint light. It is not essential to vision, since it is absent from a part of the retina identified with most ac- curate sight; this area consists of cones only. Since the rodpigmi'nt exists, in any appreciable quantity, only wlien the illumination ' is faint, and is found abiiiKhuitly in the retina of the owl and other niglitbjvds. but not at all in the rods of those animals which 'go to bed with the chickens,' it is believed to be the source of the adaptation to a faint light which the eye experi- ences aflcr being for Iwenty minutes in a dark- ened mom ('extended" Pnrkinie phenomenon). T( is very probalile that the rmla themselves con-
