Note 91 — Chapter XIX

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.