'The experiments described in this paper attempt to examine retinal "induction" by means of rotating discs. I am aware that in studying the subject thus by aid of moving chequered surfaces - that is, under intermittent excitation - time relations enter introducing complexity and preparing pitfalls for the observer. In the two well-known simultaneous contrast discs given by Helmholtz [note: Physiol. Optik. p. 413. Part II. 1860.'] the translation of surface is intended merely as a means towards the attainment of those smooth sensations - e.g. homogeneous grevs - which offer such favourable fields for contrast. The rate of translation used is one ensuring perfect fusion of the sectors. So also in those few of the beautiful series of contrast experiments by Hering in which use is made of the colour top [note: 'Pflüger's Archiv, XLI. p. 1. 1887.']' (33-34)