A Modification of Aristotle's Experiment
The generally received explanation of the illusion in Aristotle’s experiment is that two parts of the skin are being touched which in the ordinary position of the fingers have always been touched by two objects. If this is correct, we might expect to find that if two parts of the skin ordinarily touched by one object are simultaneously touched by two objects, there would be the illusive perception of one object, and such is the case. The illusion occurs most readily in a person unaware of the nature of the experiment. His eyes should be closed and his fingers crossed as for Aristotle’s experiment. If the outer sides of the crossed fingers (the ulnar side of the index and the radial side of the middle finger) are simultaneously touched by two rods, such as pencils or penholders, in many cases the observer experiences the illusion of having one rod between his fingers; “I feel as if you had a single rod going between my two fingers and touching both.” The illusion has occurred in more than half of the people with whom I have tried the experiment. It is not so easily obtained as the better known form, but this is the necessary result of several difficulties attending the modified experiment. It is difficult to make the contacts simultaneous, and several observers have noted the successive touches and judged that there were two objects; if the fingers are not touched on corresponding spots, i.e. the index or the ring finger nearer the tip than the middle finger, the illusion may not occur; again if too much pressure is exerted on the fingers, the observer may recognise that such pressure would not arise from one object. Those observers in whom the illusion does not occur, do not usually localise the touches correctly, but suppose that the two contacts are on the opposite sides of one finger.
A very interesting answer was received in one case, viz. that three touches were felt, one on each finger and a third between the fingers, and that the third contact differed from the other two in not being cold (metal rods were being used). In a note in the first volume of ‘‘Mind’’, Croom Robertson pointed out the striking illusion of spatial reference which occurs when the crossed fingers are touched one after the other, and was inclined to regard Aristotle’s illusion as secondary to this illusionary localisation in space; “we perceive the contacts as double because we refer them to two distinct parts of space.” Such an explanation would also apply to this modification, the contacts being perceived as single because we refer them to one part of space. If we cross the index and the middle fingers and hold them vertically, so that the radial side of the middle finger is uppermost, and then with close eyes successively touch the upper and lower borders, the two touches will be localised very close to each other, if not actually in the same point in space.
Aristotle’s illusion may be regarded as the analogue in the tactile sense of double vision. Here, following an unusual position of the eyes, an object is seen double because it stimulates parts of the retinae ordinarily stimulated by two objects, and the modification has also its analogue in the visual sense. When parts of the retinae ordinarily stimulated by one object are stimulated by two objects either by the convergence of the eyes to a point nearer than the object or divergence to a point beyond, there is an illusion of one object, the binocular combined image. In the case of vision however this combined image is accompanied by the two original objects, and it is this which gives especial interest to the observation where three touches were felt. Though the phenomenon has only been experienced by one observer, it supports the supposition that the single image which many observers obtain from the two contacts is a combined image analogous to the binocular combined image.