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No. 6.] position, while he gets simultaneously a third sensation of sound. It is harder to 'fix' the index at the instant the sound is caught than to note it a moment before, if the impressions come slowly and attention is ready in advance; a moment after, if the case is reversed. In the Harvard experiments, made with an improved machine, Messrs. Angell and Pierce find a preponderance of positive errors without any constant influence due to alterations of speed. Differing from both Wundt and James, they base their interpretation on the vibratory movement of attention. The experiment assumes the form of apperceptive reaction, in which the positive errors are due to the time consumed in transforming the auditory sensation into a motor act. Negative errors are referred to several causes, viz.: unconscious correction through experience become automatic; attention transferred from visual element to auditory without being shifted back, the sound occurring so soon as to be supposed simultaneous with the position just noted; effect due to hearing being more rapid than sight. Correct results are ascribed to chance arrangement of the heterogeneous factors involved, which may at any time so combine as to produce either sort of error. The mind has apparently no criterion by which to distinguish coincidence of disparate stimulations from rapid sequence of the same.

Though both the Young-Helmholtz theory of light- and color-vision and the hypothesis of Hering count many adherents,—the former among physiologists, the latter principally, perhaps, among pathologists,—the inadequateness of both to a full explanation of the phenomena has long been recognized in psychology. As early as 1874, Wundt (who, curiously enough, is not referred to in Mrs. Ladd-Franklin's paper, although Donders and Göller find mention) had propounded a periodicity-theory in place of the three- and four-component hypotheses. Mrs. Ladd-Franklin, regarding these latter as the only alternatives, prefers the former of them, on the ground of Konig's and Dieterici's results: in other respects the theory which she proposes most nearly resembles that of Bonders. Two kinds of "molecules" are assumed to exist in the photochemical substances of the retina: gray molecules, which alone occur in the totally color-blind retina, at the periphery of the normal retina, and (probably) in the eyes of many of the lower animals,—and the dissociation of which is the cause of the sensation of gray; and color-molecules, which have arisen by differentiation from these, in that the atoms of their external layer have become grouped in three directions at right angles to one another. The writer claims that