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 he 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 ell ect of retinal fatigue is known as the negative 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 this 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. ff 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 perceived by a part of the retina which has become more sensitive from exposure to rays coming from a black ob ject; the latter surface appears dull because seen with a portion of the retina whit+ has been fatigued by exposure to red rays. See Arrim I MACE.