Especial interest attaches to the colored im ages obtained from intensive stimulation with white light. ('lose 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 known technically as the flight of colors. 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. green, and violet stimuli (the part-stimuli contained in the white 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 colors, 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 it, spectral components. We must remember, also, that "white" light is never quite colorless; theme is always some tingeof 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-o•ange disk upon a sheet of white paper and fixate it monoc ularl• for five or ten seconds. Then blow away the disk, close the stimnlated eye. open the unstimuiated one and fixate the white ground. You see at first a pile-yellowish image. Then the field darkens and a blue negative image makes its appearance. Presently the ground clears and the yellowish patch comes once more. Then the white darkens again and the blue image recurs. The darkening is due to retinal rivalry: 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 unstinrulated eve, is an after-effect of the or ange stimulation, but an after-effeet 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 visual appa ratus. Consult: 11. von Helmholtz, Physiolo gische nptilc (Hamburg, 1S911) E. Hering, Zur Lehre rout. Lichtsinne (Vienna, 1S75) : O. Kuelpe, Outlines of Psychology (London, 1895) ; E. B. Titchener, Experimental Psychology (New York, 1901).