[17] _Die Schein-Bewegungen_, von Professor Dr. J.I. Hoppe (1879); _cf._ an ingenious article on "Optical Illusions of Motion," by Professor Silvanus P. Thompson, in _Brain_, October, 1880. These illusions frequently involve the co-operation of some preconception or expectation. For example, the apparent movement of a train when we are watching it and expecting it to move, involves both an element of sense-impression and of imagination. It is possible that the illusion of table-turning rests on the same basis, the table-turner being unaware of the fact of exerting a certain amount of muscular force, and vividly expecting a movement of the object. [18] _Physiologische Optik_, p. 316. [19] It is plain that this supposed error could only be brought under our definition of illusion by extending the latter, so as to include sense-perceptions which are contradicted by reason employing idealized elements of sense-impression, which, as Lewes has shown (_Problems of Life and Mind_, i. p. 260), make up the "extra-sensible world" of science. [20] An ingenious writer, M. Binet, has tried to prove that the fusion of homogeneous sensations, having little difference of local colour, is an illustration of this principle. (See the _Revue Philosophique_, September, 1880.) [21] Even the fusion of elementary sensations of colour, on the hypothesis of Young and Helmholtz, in a seemingly simple sensation may be explained to some extent by these circumstances, more especially the identity of local interpretation. [22] The perception of lustre as a single quality seems to illustrate a like error. There is good reason to suppose that this impression arises through, a difference of brightness in the two retinal images due to the regularly reflected light. And so when this inequality of retinal impression is imitated, as it may easily be by combining a black and a white surface in a stereoscope, we imagine that we are looking at one lustrous surface. (See Helmholtz, _Physiologische Optik_, p. 782, etc., and _Populaere wissenschaftliche Vortraege_, 2tes Heft, p. 80.) [23] The conditions of the production of these double images have been accurately determined by Helmholtz, who shows that the coalescence of impressions takes place whenever the object is so situated in the field of vision as to make it practically necessary that it should be recognized as one. [24] These illusions are, of course, due in part to inattention, since close critical scrutiny is often sufficient to dispel them. They are also largely promoted by a preconception that the event is going to happen in a particular way. But of this more further on. I may add that the late Professor Clifford has argued ingeniously against the idea of the world being a continuum, by extending this idea of the wheel of life. (See _Lectures and Essays_, i. p. 112, _et seq._)