[12] For a confirmation of the view adopted in the text, see Professor Bain, The Senses and the Intellect, Part II. ch. i. sec. 8; Herbert Spencer, Principles of Psychology, vol. i. p. 234, et passim; Dr. Ferrier, The Functions of the Brain, p. 258, et seq.; Professor Wundt, op. cit., pp. 644, 645; G. H. Lowes, Problems of Life and Mind, vol. v. p. 445, et seq. For an opposite view, see Dr. Carpenter, Mental Physiology, fourth edit., p. 220, etc.; Dr. Maudsley, The Physiology of Mind, ch. v. p. 259, etc.

[13] See note, p. 22.

[14] Touch gives much by way of interpretation only when an individual object, for example a man’s hat, is recognized by aid of this sense alone, in which case the perception distinctly involves the reproduction of a complete visual percept.  I may add that the organ of smell comes next to that of hearing, with respect both to the range and definiteness of its simultaneous sensations, and to the amount of information furnished by these.  A rough sense of distance as well as of direction is clearly obtainable by means of this organ.  There seems to me no reason why an animal endowed with fine olfactory sensibility, and capable of an analytic separation of sense elements, should not gain a rough perception of an external order much more complete than our auditory perception, which is necessarily so fragmentary.  This supposition appears, indeed, to be the necessary complement to the idea first broached, so far as I am aware, by Professor Croom Robertson, that to such animals, visual perception consists in a reference to a system of muscular feelings defined and bounded by strong olfactory sensations, rather than by tactual sensations as in our case.

[15] It may be said, perhaps, that the exceptional direction of attention, by giving an unusual intensity to the impression, causes us to exaggerate it just as in the case of a novel sensation.  An effort of attention directed to any of our vague bodily sensations easily leads us to magnify its cause.  A similar confusion may arise even in direct vision, when the objects are looked at in a dim light, through a want of proper accommodation. (See Sir D. Brewster, op. cit., letter i)

[16] They might also be distinguished as objective and subjective illusions, or as illusions a posteriori and illusions a priori.

[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.