An optical illusion is discovered in a single instance of the phenomenon.  We are aware of the illusion almost immediately.  But in the case of most of the illusions of touch, a large number of experiments is often necessary in order to reveal any approximately constant error in the judgments.  Nevertheless, it seems to me that the factors that influence our judgments of visual space, though their effects are nearly always immediately apparent, are of no more vital significance for the final explanation of the origin of our notion of space than the disturbing factors in our estimations of tactual space whose effects are not so open to direct observation.

The present investigation has for its main object a critical examination of the tactual illusions that correspond to some of the well-known optical illusions, in the hope of segregating some of the various disturbing factors that enter into our very complex judgments of tactual space.  The investigation has unavoidably extended into a number of near-lying problems in the psychology of touch, but the final object of my paper will be to offer a more decisive answer than has hitherto been given to the question, Are the optical illusions also tactual illusions, or are they reversed for touch?

Those who have given their attention to illusions of sight and touch are rather unequally divided in their views as to whether the geometrical optical illusions undergo a reversal in the field of touch, the majority inclining to the belief that they are reversed.  And yet there are not wanting warm adherents of the opposite view.  A comparison of the two classes of illusions, with this question in view, appears therefore in the present state of divergent opinion to be a needed contribution to experimental psychology.  Such an experimental study, if it succeeds in finding the solution to this debate, ought to throw some further light upon the question of the origin of our idea of space, as well as upon the subject of illusions of sense in general.  For, on the one hand, if touch and sight function alike in our judgment of space, we should expect that like peripheral disturbances in the two senses would cause like central errors in judgment, and every tactual analogue of an optical illusion should be found to correspond both in the direction of the error and, to a certain extent, quantitatively with the optical illusion.  But if, on the other hand, they are in their origin and in their developed state really disparate senses, each guided by a different psychological principle, the illusion in the one sense might well be the reverse of the corresponding illusion in the other sense.  Therefore, if the results of an empirical study should furnish evidence that the illusions are reversed in passing from one field to the other, we should be obliged to conclude that we are here in the presence of what psychologists have been content to call the ‘unanalyzable fact’ that the two senses function differently under the same objective conditions.  But if, on the contrary, it should turn out that the illusions are not reversed for the two senses, then the theory of the ultimate uniformity of the psychical laws will have received an important defence.