[91] The converse illusion is hard to bring about. The points _a_ and _b_, being normally in contact, mean to us the same space, and hence it might be supposed that when simultaneously touched, as by a pair of callipers, we should feel but one object, whilst us a matter of fact we feel two. It should be remarked in explanation of this that an object placed between the two fingers in their normal uncrossed position always awakens the sense of _two contacts_. When the fingers are _pressed together_ we feel _one object_ to be between them. And when the fingers are crossed, and their corresponding points _a_ and _b_ simultaneously _pressed_, we do get something like the illusion of singleness--that is, we get a very doubtful doubleness. [92] Purkinje, Mach, and Breuer are the authors to whom we mainly owe the explanation of the feeling of vertigo. I have found (American Journal of Otology, Oct. 1882) that in deaf-mutes (whose semi-circular canals or entire auditory nerves must often be disorganized) there very frequently exists no susceptibility to giddiness or whirling. [93] The involuntary continuance of the eye's motions is not the only cause of the false perception in these cases. There is also a true negative after-image of the original retinal movement-sensations, as we shall see in Chapter XX. [94] We never, so far as I know, get the converse illusion at a railroad station and believe the other train to move when it is still. [95] Helmholtz: Physiol. Optik, 365. [96] Cf. Berkeley's Theory of Vision, §§ 67-79; Helmholtz: Physiologische Optik, pp. 630-1; Lechalas in Revue Philosophique, xxvi. 49. [97] Physiol. Optik, p. 602. [98] It seems likely that the strains in the _recti_ muscles have something to do with the vacillating judgment in these atropin cases. The internal recti contract whenever we accommodate. They squint and produce double vision when the innervation for accommodation is excessive. To see singly, when straining the atropinized accommodation, the contraction of our internal _recti_ must be neutralized by a correspondingly excessive contraction of the external _recti_. But this is a sign of the object's recession, etc. [99] American Journal of Psychology, i. 101 ff. [100] Romanes, Mental Evolution in Animals, p. 324.