Page:The New International Encyclopædia 1st ed. v. 01.djvu/242

AFTER-IMAGES. part of the retina upon which the green falls will become gieen-adapted, and tlierefore red-dis- posed or red-sighted. Hence, if I 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 briglit 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 gi'ay 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 tlie 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. Theoreticalh' important is the fact that a contrast-color (see Contrast) set up in the neighborhood of the stimulus is efTcctive 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 stimulvis be 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 Visual Sexs.ition.) 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 inteiwal, somewhat longer than the first; and, finally, by the dark-green negative image. The usual explanation of this positive 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 accovmt 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- scribed above as "blank") may, under certain circumstances, be filled by a positive and comple- mentary image. 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. The dull red is, evidently, not a direct continuation of the red of the stim- ulus. No satisfactory theory is as yet forth- coming.

Especial interest attaches to the colored im- ages obtained from intensive stimulation with white light. Close your eyes and keep them closed until there is no trace of previous stim- ulation (no colored after-image) on the dark field. Then fixate for some twenty seconds the middle bar of a window which looks out upon a brilliantly white sky. Close your eyes again, and note the development of the after-image on the dark field. You see a color sequence, which is knoW'U technically as the flight of eolors. The current explanation of the phenomenon is that the white light of the sky is broken up into its physical components, in somewhat the same way as a ray of light passing through a prism is broken up into the series of spectral colors; and that the retinal excitations corresponding to the red, gieen, and violet stimuli (the part-stimuli contained in the w-hite light) are not exactly co- incident, but overlap in time, so that now the one and now the other shows itself in the after- image. It is, however, noteworthy that the flight of eolors, under conditions of exact ob- servation, shows unmistakable evidence of two overlapping complementary series. The sequence is: a momentary positive image; then, after fluctuations, a blue, a green, a yellow, a red (at this stage the image becomes negative), a blue, and a green image. We have, that is, the series blue-yellow-blue and the series green-red-green laid over one another ; there is clear indication of antagonism or complementarism, but none of a general breaking up of the white light into its spectral components. We must remember, also, that "white" light is never quite colorless; there is always some tinge of color in diffuse daylight. The facts point to the validity of an "antag- onistic" theory of visual sensation (q.v.). (3) We may note, finally, the existence of a binocular or transferred after-image. If one eye be stimulated, under suitable conditions, a faint, positive image appears in the field of the other unstimulated eye. Lay a bright red-orange disk upon a sheet of white paper and flxate it monoe- ularly for five or ten seconds. Then lilow away the disk, close the stimulated eye, open the un- stimulated one and fi.xate tlie white ground. You see at first a pLile-yellowish image. Then the field darkens and a blue negative image makes its appearance. Presently the ground clears and the yellowish j^atch comes once more. Then the white darkens again and the blue image recurs. The darkening is due to retinal rivahy: the dark field of the closed (stimulated) eye is superposed upon the bright field of the open (unstimulated) eye. The blue image is the nega- tive after-image belonging to the dark field, i.e., to the originally stimulated eye; its ap- pearance requires no explanation. On the other hand, the faint yellowish image belongs to the unstinuilated eye. is an after-effect of the or- ange stimulation, but an after-effect that differs entirely from the after-effect in the stimulated eye, and that has been transferred to the eye which was not exposed to the stimulus. Its existence points to a close functional inter-rela- tion between the two halves of the vismil appa- ratus. Consult: H. von Helmholtz. Phiisioh'- gische Optilc (Hamburg, 1806) ; E. Hering, Zur Lehre vom Lichtsinne (Vienna, 187S) ; 0. Kuelpe, Outlines of Psychology (London, 1805) ; E. B. Titchener, Experimental Psychology (New York, 1901).

AFZE'LIUS, Sw. pron. Af-tsa'li-oos, Adam (1700-1837). A Swedish naturalist, u puiiil of Linna'us, whose autobiography he afterward edited. He was professor in the t'nivcrsity "f Upsala. He studied the flora of West Africa from 1702 to 1704, and wrote many botanical papers for the Danish Royal Academy and the