An Optical Phenomenon

IN physiological laboratories several routine experiments are in use for demonstrating phenomena of the kind described by Capt. C. J. P. Cave (NATURE, December 13, p. 284). These phenomena all support Hering's theory of the reciprocal after-effects of stimulation (see W. H. R. Rivers in Schäfer's “Text-book of Physiology,” vol. ii., pp. 1146–47, especially fourth paragraph, p. 1147). The seat of illusion begins at the retina, but (as hinted in my letter on p. 165, November 1) involves all the intricate labyrinth of nerve tissue from the retina to the highest sensory and motor centres.