[47] I refer to the experiments made by Exner, Wundt, and others, in determining the time elapsing between the giving of a signal to a person and the execution of a movement in response.  “It is found,” says Wundt, “by these experiments that the exact moment at which a sense-impression is perceived depends on the amount of preparatory self-accommodation of attention.” (See Wundt, Physiologische Psychologie, ch. xix., especially p. 735. et seq.)

[48] Quoted by Helmholtz, op. cit., p. 626.

[49] When the drawing, by its adherence to the laws of perspective, does not powerfully determine the eye to see it in one way rather than in the other (as in Figs. 5 to 7), the disposition to see the one form rather than the other points to differences in the frequency of the original forms in our daily experience.  At the same time, it is to be observed that, after looking at the drawing for a time under each aspect, the suggestion now of the one and now of the other forces itself on the mind in a curious and unaccountable way.

[50] Ueber die phantastischen Gesichsterscheinungen, p. 45.

[51] Another side of histrionic illusion, the reading of the imitated feelings into the actors’ minds, will be dealt with in a later chapter.

[52] In a finished painting of any size this preparation is hardly necessary.  In these cases, in spite of the great deviations from truth in pictorial representation already touched on, the amount of essential agreement is so large and so powerful in its effect that even an intelligent animal will experience an illusion.  Mr. Romanes sends me an interesting account of a dog, that had never been accustomed to pictures, having been put into a state of great excitement by the introduction of a portrait into a room, on a level with his eye.  It is not at all improbable that the lower animals, even when sane, are frequently the subjects of slight illusion.  That animals dream is a fact which is observed as long ago as the age of Lucretius.

[53] This kind of illusion is probably facilitated by the fact that the eye is often performing slight movements without any clear consciousness of them.  See what was said about the limits of sensibility, p. 50.

[54] Mental Physiology, fourth edit., p. 158.

[55] In persons of very lively imagination the mere representation of an object or event may suffice to bring about such a semblance of sensation.  Thus, M. Taine (op. cit., vol. i. p. 94) vouches for the assertion that “one of the most exact and lucid of modern novelists,” when working out in his imagination the poisoning of one of his fictitious characters, had so vivid a gustatory sensation of arsenic that he was attacked by a violent fit of indigestion.

[56] Mentioned by Dr. Carpenter (Mental Physiology, p. 207), where other curious examples are to be found.

[57] See Annales Medico-Psychologiques, tom. vi. p. 168, etc.; tom. vii. p. 1. etc.