If one eye only be stimulated, an after image may appear in the unstimulated eye. Four hypotheses have been offered to explain this transfer of the image from one eye to the other: (1) The appearance is a phenomenon of binocular contrast. When one eye is stimu lated by a bright colored light, and the other eye is stimulated by a very little gray light or is protected from all light, the contrasted color may be seen in the unstimulated eye during the time of stimulation, and this may leave an after-image in that eye. (2) A second hypoth esis is that the eyes are accustomed to func tion together, and whatever affects one retina affects the other also. This may be considered as a modified form of the first hypothesis. (3) Another hypothesis is that the after-image has its seat in the centres in the brain, not in the end organ or retina, and that it may be seen in whichever eye is open. This hypothesis seems to be overthrown by the fact that an electrical stimulation of the optic nerve pro duces a sensation like that produced by a flash of light, but no after-image follows. Another
fact difficult for this hypothesis to explain is that if one eye be stimulated the after-image appears in the other eye only in case that eye be well darkened. (4) A fourth hypothesis is that the transfer of the after-image is not real but only apparent. In support of this hypothe sis it has been found that when portion of the right eye which corresponds to the blind spot of the left eye was stimulated* (Franz). there was an apparent transfer of the image to the left eye; also if the unstimulated eye be disturbed or interfered with during the course of the after-image no change in the image may be observed, whereas if the stimulated eye be interfered with the image disappears. Consult Fechner. 'Elemente der Psycho-physik' ; Franz, S. I., (Monograph Supplement, Vol. III. No. 2 1899, Psych. Rev.); Titchener, E. B., 'Ueber binocular Wirkungen monocular Reize.) See EYE; Vtsum.