Page:The New International Encyclopædia 1st ed. v. 20.djvu/227

* VISION. 179 VISION. distinctly visible for a considerable time after the Inininous stimulus has ceased to act. Thi.s positive afterimage may be exijcrienced by look- ing at a bright object, such as a gas jet, for a sliort pcrio<l and then closing the eyes, when a brilliant image will lie perceived for some time. A luminous impulse of very short duration is sufficient to produee a eom])lete retinal imjires- sion; thus it has been calculated that an electric 1 spark lasting ^ of a second is distinctly 20,0HU,Ci00 visible. This short duration refers only to the time necessary to excite? the sensibility of the retina; the time required for perceiving the sen- sation is much longer, since the latter process in- cludes passage of the visual sensation through the optic nerve (lasting at least ^rnrj- of a sec- ond), transmission through the intracerebral visual paths, and perception by the visual cortical area of the brain. Consequently by the time the image of an electric spark is perceived, it has already come to an end. This explains the fact that bodies in rapid motion appear as if at rest, when seen by a very brief illumination, such as an electric spark. For the same reason, the brief closure of the lids in winking causes no interference with sight, since the visual impres- sion of external objects persists beyond the time required for rapidly closing and opening the lids. See Duration. Fatigue of the Retina. The sensibility of the retina becomes diminished when the eye is ex- posed to continued impressions. If one eye be covered with a dark shade and the other be fixed upon a uniformly illuminated white surface, the latter becomes less brilliant after a time, owing to the retinal fatigue; if we then cover the eye previously in use and look at the white surface with the other, it will appear brighter because the retina previously shaded will have acquired greater relative sensibility. If we look steadily at a black disk one or two inches in diameter, placed upon a sheet of white paper, and then re- move the disk and continue to look upon the paper, the latter will present a bright spot cor- responding to the previous position of the black disk. This example of the eflect of retinal fatigue is known as the negative after-image ; the por- tion of the retina which was originally exposed only to the rays from the black disk with its comparative absence of light has become more sensitive, and consequently the portion of the paper corresponding to tliis sensitive area ap- pears brighter; while the parts of the retina impressed by rays from the white paper become less sensitive and as a result this portion of the paper appears duller. Parts of the retina which have suffered fatigue by continued impression of color are more sen- sitive to rays of the complementary color. If we place a piece of red paper upon a white ground and look at it steadily, a green after- image is seen when the colored paper is removed. If the eyes be fixed upon a piece of black ribbon placed upon a red cloth, the negative image seen upon withdrawing the ribbon will be a deep red color, the rest of the cloth appearing to be a dull brown ; the former area is [lerceived by a part of the retina which has become more sensitive froai exposure to rays coming from a black ob- ject; the latter surface appears dull because seen with a portion of the retina which has been fatigued by exposure to red rays. See Afteb- Imaoe. Co.n'TKASt. Wlien after being exposed lo one portion of the field the eye seeks another, the visual sensation will be modified by the amount of fatigue excited by that portion of the field upon which the eye has been fixed; the effect of (his combination is to produce the phenomena of contrast. As a result of contrast, every part of the field of vision apjiears darker near a lighter portion and lighter near a darker por- tion. In Fig. 3, an example of contrast, a inun- ber of black squares are se])arated by white bands ; the points of intersection of the latter ap- pear darker than the rest of the white bands. See CONTKAST. Fig. 3. Illustrating the phenomenon of contradt. Irradiation. The refractive apparatus of the eye, though wonderfully constructed, presents some imperfections. Owing to the latter, points of light are not focused with absolute accuracy, and consequently produce small circles of dis- persion on the retina. On this account, bright objects produee images with imperfectly defined outlines which appear larger than when objects of the same size are feebly illuminated ; this phenomenon is known as irradiation. A familiar example is seen in Fig. 4. A white square upon a dark field appears larger than a black square of identical size upon a white background, be- cause the white surface appears to encroach upon the dark portions of the field. Fig. 4. Illustrating the phenomenon of irradiation. Space Perception. Rays of light proceeding from a certain point in the visual field are