VISION (Lat. see).—Lit. the faculty of seeing; that power by which we perceive the forms and color of objects through the sense of sight.
The study of the eye, and the analogy it bears to the photographic camera and lens, is one that is full of interest to the photographic student. The eye is a natural camera obscura lens and diaphragm. It is placed in a bony cavity termed the orbit, and maintained in position by the muscles which serve to move it, by the optic nerve, the conjunc tiva, and the eyelids. It is about the same size with most per sons, for, although it appears to differ very greatly with many, it is in reality the varying aperture of the eye-lids that makes it appear larger or smaller.
The shape of the eye is that of a spheroid, the curvature of which is greater in the anterior than in the posterior part. It is composed of the following parts:—The sclerotica, or white of the eye, which serves for the attachment of the muscles; a trans sparent medium in front of the eye, through which we see, termed the cornea, a tough, velvety membrane, termed the choroid, on the inner surface of which is a dark substance preventing in ternal renection. The retina is the inmost coat of all, and is composed of layers of nerve-cells, which transmit their im pressions to the brain Via the optic nerve which, communicating directly with the brain, is the immediate seat of vision.* In the center of the iris of the eye is a circular opening called the pupil Behind the pupil and iris is the crystalline lens, a firm and transparent body through which the rays of light pass on their way from the pupil to the retina. It is a single, though not a simple, biconvex lens, about half-an-inch in diameter, and with a little over half-an-inch focus. The posterior chamber is filled with the vitreous humor, which shapes the whole constructions.
From this description of the structure of the eye its resemblance to the photographic camera will be apparent. The pupil is the aperture, the crystalline the condensing lens, and the
retina the screen on which the image is formed.
The image thrown upon the retina is inverted, con sequently a natural ques tion arises, Why do we not see objects upside down ? The explanation is that " the image simply causes a stimulation of the optic nerve, which produces a change on some part of the brain, and it is only of this change that we are aware, and not of the image it self.
The optic angle is the angle formed between the principal optic axes of the two eyes when they are directed toward same object. This angle is, of course, smaller the greater the distance of the object.
The visual angle is the angle under which an object is seen. Taking the same distance, this angle increases with the size of the object, and with the same sized object it decreases as the distance increases. It is therefore evident that objects appear smaller the greater the distance they are from the eye.
A single eye sees most distinctly any point situated in its principal axis, and less distinctly other points towards which it is not directly looking, but which are still within its angle of vision. It is able to judge the direction of any such point, but unable by itself to estimate its distance. Of the distance of an object it may be able to judge by loss of color, decrease in magnitude, indis tinct outline, etc.; but if the object be near, the single eye is not infallible with these aids.* When both our eyes are directed upon one single point we then have a power of judging the distance of that point in comparison with that of any other point, and this power we seem to gain by the sense of the amount of effort required to cause the optical axes to converge upon one point and upon another.