(1) Introduction. Colored stimulation of one retina evokes, under certain conditions, a color-sensation in the other. This has been explained as due to binocular contrast (Fechner, Wundt, Helmholtz, Béclard); to a direct excitation of the second retina (Chauveau; denied by Charpentier); to the central origin of after-images in general (Parinaud, Binet), or of the binocular after-image in particular (Ebbinghaus). (2) Experimental results. The phenomenon is to be ascribed to physiological excitation of the second retina (cf. Ramón y Cajal) . The. secondary after-image differs from the primary: (a) as being of less absolute duration; (b) as being of relatively different duration in its successive phases; (c) in brightness (at least in the negative and complementary stages of both) . It appears in circumstances which preclude the possibility of retinal rivalry (ct. Exner, Kleiner, Delabarre). Experiments carried out with a unilaterally color-blind subject confirmed its independent existence. (3) Theory. The secondary after-image can be subsumed to Fechner's theory. Positive and complementary after-images are explained as "exhaustion phenomena, which have not yet reached the value of the difference limen in the negative direction, as regards their brightness-coefficients." The fact, established in the course of the investigation, that color-blind constituents of the one half of the optical apparatus can conduct to the other half an excitation of the quality to which they themselves are insensible, cannot be turned to account for the decision of the question of the peripheral or central seat of color-blindness. The experiments indicate that the physiological basis of contrast is different from that of after-image, if Fechner's theory of the latter is maintained; but they make no positive contribution to a theory of contrast-phenomena.
(1) Preliminary remarks. Color-sensation is not an explicit function of wave-length. For, firstly, the same sensation is occasioned by rays of quite different refrangibility, if the brightness-relations are suitable. Secondly, the law of relativity comes into operation. Thirdly, there is no such thing as homogeneous light: the collimator slit of the spectroscope is always wider than the wave-length of the rays employed. This latter fact helps to explain the law of mixture of non-complementary colors, and the position of green in the series of spectral colors,
