THE FORMATION OF IMAGES. Diffuse light gives rise merely to the impression of luminosity. In order to see objects distinctly, light must he col lected so as to produce a sharply defined image upon the retina; this process constitutes focus ing. Rays from different points of the object must be focnsed at corresponding points on the retina, and every radiant point of the object must have a corresponding focal point in the image (Fig. 1). If, for instance, the usually transparent cornea be rendered opaque by some disease, the person will still perceive light, but will he un able to recognize objects.
The eye serves as au optical instrument which receives 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 resemblance. Thus, the camera incloses a dark chamber, the walls of which are blackened so as to prevent reflection 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 fowls of the rays.
Compared with the camera, the eye possesses all these parts, arranged, of course, in a nmeh more delicate and wonderful manner. It is a (lark chamber, lined with the black pigment of the retina and choroid, presenting an opening in front for the admission of light (the pupil). and provided 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 camera by a series of stops or diaphragms, are excluded in the eye by a colored curtain, the iris. The opening in the iris, the pupil, en larges or diminishes in size according to various conditions presented. In a bright light and when the eye is focused upon near objeets, the pupil eontracts; 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 and requires different diaphragms for various changes in the distance of the object and the intensity of illumination.
AccnmooaAY1 ON The focusing apparatus of the eye is of wonderful construction. When we look at a distant objeet, the lens is kept that by the tension of its capsule and its suspensory ligament. and is consequently' less refractive; rays proceeding from it distant object being less divergent than from a near source, less power is required to focus such rays upon the retina. NVhen the eye is fixed upon a near objeet, the rays coining from the latter being more divergent, greater refractive power is necessary in order to produce a distinct image upon the retina. Cutter 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 by its contrac tion relaxes the capsule of the lens, 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 objects, 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. Accommodation 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 (preshyopia or old sight), which requires for its correction the use of spectacles containing con vex lenses.
TuE RETINA. TiliS is the portion of the eye sensitive to light waves, and is described under EYE. The particular part thus sensitive is the layer whieh is called the rods and cones. This layer is composed of great numbers of Juinute rod-shaped and eone-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. Stimulation of the retina, in whatever way produced. is accompanied by the sensation 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 optic nerve there is the sensation of flashes of light.
In the out portion of the rods, though not in the cones, is a purplish-red matter known as the 'visual purple. 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 l'iS1011, Since it is absent front a part of the retina identified with most ac curate sight: this area consists of cones only. Since the rod-pigment exists. in any appreciable quantity. only when the illumination is faint, and is found abundantly in the retina of the owl and other night birds. but not at all in the lolls of those animals which '.go to bed with the chiekens,' it is believed to be the source of the adaptation to a faint light which the eye experi ences after being for twenty minutes in a dark ened room (`extruded' Purkinie phenomenon). It is very probable that the rods themselves eon stitute an apparatus for impressions in black and white only, and are wholly insensitive to color.