Page:The New International Encyclopædia 1st ed. v. 20.djvu/225

* VISION. 177 VISION. Btitute an apparatus for impressions in black and wliito only, and are vvliolly insensitive to color. The Field oi- Vision. The sensitiveness of the retina varies in ditierent parts. The most sen- sitive portion corresjionds to the posterior pole of the eyeball, a reyion which is called on ac- count of its color the yellow spot or itnicuUi lultm, and on account of its special endowment the reijion of most distinct vision; this area presents in its centre a depression called the fovea. The fovea has no nerve Ul)res, blood-vessels, or reliiial elements e.xcepting cones; oven the rods are ab- sent. The niiiciila (and still more markedly the fovea) serves to receive all those luminous sen- sations which give rise to distinct impressions of tision. When we wish to see anything distinctly, we turn the eyeballs toard the object, so that the image will fall upon this spot. Other parts of the retina are much less sensitive to form and color, but not to variations in brightness. Vision can, therefore, be divided into (1) cen- tral or distinct, when the image of the object falls on the macula, and (2) peripheral, wlicn the image falls on some other part of the retina. The space before the eye within which objects are seen, even though indistinctly, constitutes the visual peld: its dimensions for each eye are: Externally or toward the temple, 90°, above 50°, below 70°, and internally or toward the nose, 60° ; these represent the limits for white light, the fields for colors being less extensive. Though not adapted for distinct vision or for color- vision, the peripheral field is of the greatest use in giving us knowledge of the existence of objects placed outside of the direct line of vision ; with- out it our surroundings would have the apjiear- ance which we get when we look through a tube. Though we see indistinctly with peripheral vision, eccentric portions of the retina are more sen- sitive to differences of light, irrespective of form and color, than the macula; hence in a very dim light we can often see an object better when it is outside of the line of direct vision, that is, ■when we are not looking directly at it. The Blind Spot. The region corresponding to the entrance of the Ojitie nerve into the eyeball is occupied by nerve fibres exclusively and pre- sents an absence of all other retinal elements. This area constitutes a blind spot in the field of vision ; though not generally noticeable, its ex- istence can be demonstrated in the following manner. A cross and a round spot are drawn Flfi. 2. For u5e in the experiment to demonstrate the existence of the blind spot in the field of vision. upon a sheet of paper, two or three inches apart (Fig. 2). the left eye is closed and the right eye fixed upon the round spot: if now the sheet be moved forward and backward before the exposed eye, the cross will disa])pear from view at a certain distance (about 3}„ times the distance between the two figures) whenever its image falls upon the blind spot. See Blind Spot. The retina is supplied with blood-vessels which enter the eyeball with the optia nerve and, br:inching into smaller ramifications, spread out in the inner layer of the retina. Since they oc- cupy a plane in. front of that of the sensitive layer (rods and cones), they must cast a visible shadow; we are not usually conscious of these shadows, but we can be made aware of their ex- istence by the following experiment. If, in a dark room, which serves to make the retina sen- sitive, after a time, to faint illuminatitm, we keep a lighted candle moving upward and down- w;inl on one side of the eye, these shadows will become visible as beautiful arborescent lines. The Ophthalmoscope. All details of the inner aspect of the retina become visible when the eye is explored with the ophthalmoscope, an in.stru- nient invented by Helndioltz in 1851, the essential p:irt of which is a perforated mirror which serves to rellect light into the interior of the eye. Interphetation of Retinal Images. The stimuli aflecting the fibres of the ojitie nerve whenever the rods and cones of the retina are ex- cited by the vibrations of the huniniferous ether'' are conveyed to the brain, where they determine changes in the cortical cells. The accompanying impressions are then referred by consciousness to definite positions in external space. Each point in a retinal image is referred to an external point in the line of direction (line through the nodal point of the eye) and consequently to the opposite side of the field of vision; objects seen to the right cast their images upon the left side of the retina, those occupying the left side of the field upon the right side of the retina. Though usually excited by stimuli from without, the retina may also be affected by luminous sen- sations independent of external causes, or the cerebral centre of sight may be the seat of excita- tion, producing impressions which are in every case referred to external space, thus giving rise to delusive visual phenomena (hallucinations). See Distance; Locality; Movement, Percep- tion OF. Corresponding Points. Every part of the ret- ina has its corresponding point in the field of vision; this is found by projecting a line from the retinal point through the nodal j)oint of the eye, situated near the posterior pole of the lens. As a result of this law, it follows that the retinal image nuist be reversed as compared to the ex- ternal object, the upper portion of the field cor- responding to the lower part of the retina, and the lower portion of the field to the upper part of the retina. There has been much discussion as to how it is that we see objects right side up when the retinal image is inverted, but the be- lief that there is here anything to be explained is due to a misconception. We have no con- sciousness of either the retinal image or the retina, any more than of the crossing of the fibres of the optic nerve in the optic chiasm. Visual sensation follows only upon an excita- tion of the cortex of the brain, and if we were conscious of any part of the mechanism of seeing, it ought to be of the (chemical) reproduction of the image upon the visual cortical area. To sup- pose that we know anything about the physiolog- ical basis of our sensations, subjectively, is to commit what Professor .James has called the psychologist's fallacy — the fallacy of thinking