AF'TER-IM'AGES. Retinal images which ap pear after time eye has been removed from some illuminated object. When we light our lamp in the evening we are distinctly conscious that the illumination has a reddish-yellow tinge. As time goes on however. we lose the color; the paper ou which we write seems to he as white as the same paper seen in diffuse daylight; our eyes have become adapted. or have grown used. to their surroundings (general adaptation). The law of adaptation is that all brightnesses tend toward a middle gray, and all colors toward neutrality. Adaptation leaves an after-effect, which is termed disposition. A yellow•adopted eye is disposed to the complementary color. or blue•sighted: all the Fellows that it sees tend toward gray, and all other colors take on a tinge of the complementary blue. See CONTRAST VISI SENSATION, Adaptation may be /oral, as well as general. Suppose. e.g., that 1 fixate steadily a green disk seen upon an extended white ba•kground. The part of the retina upon which the green falls will become green-adapted, and therefore red-dis posed or red-sighted. Pence. if 1 presently re move the green disk I shall see a subjective red disk in its place. This red, the after-effect of local adaptation, is termed (1) the negative after-image. The color and brightness of the after-image are always complementary to the color and brightness of the preceding stimulus; a dark-blue stimulus gives a bright yellow after image, and a bright yellow stimulus a dark-blue after-image. if one stares for half a minute at a window that gives upon a bright gray sky or a snowy landscape, and then turns one's eyes upon a gray screen or wall, one sees an after image window with white bars and black panes. In general, the vividness and duration of the neg ative after-image depend upon the intensity and duration of the stimulus which evokes it, and on the brightness of the surface upon which the after-image is projected for observation. It is probable, although the point is still disputed, that the course of the after-image is intermit tent. not continuous. Theoretically important
is the tact that a contrast-color (see CoNrnAsr) set up in the neighborhood of the stimulus is effective in the after-image; thus, a disk of red paper seen on a gray background, and giving a narrow green ring of marginal contrast, ap pears in the after-image as a green disk sur rounded by a distinct reddish halo. (2) If the original stimulus he very strong and of brief duration, it may give rise to what has been called the positive after-image, a subjective phenomenon in which the stimulus-sensation is reproduced, only with diminished brightness and saturation. (See VistTAL SENSATION.) Thus, a flash of brilliant red light would be followed, first, by a brief, but noticeable, blank interval; then by the positive after-image, a duller and pinker red; then by a second interval. somewhat longer than the first; and, finally, by the dark-green negative image. The usual explanation of this posit ive image is that the physiological effect of stimulation persists for some time after the physical stimulus itself has ceased to operate; the sensation, therefore, outlasts the stimulus, remaining the same in kind throughout its course. This account is, however, as inadequate as is the theory which would account for the negative after-image on the score of retinal fa tigue. It. is disproved by the single fact that the short interval which elapses between stimu lus and positive image (the first interval de seribed above as "blank") may, under certain circumstances, he filled by a positive and comple mentary i»111 fIC. Thus, if a glowing red point be moved slowly to and fro in the dark, one sees first a trail of red light (due to the stimulus and its direct. after-effect), and then a bright (positive) green streak. Then should follow, if the series is complete, the positive image proper, a dull red, the second blank interval, and the negative green image. Tlw dull red is, evidently, not a direct continuation of the red of the stim ulus. No satisfactory theory is as yet forth coming.