THE PERCEPTION OF THINGS. 87 



The forefinger-toucli seems higher, though the finger is 

 really lower ; the second-finger-touch seems lower, though 

 the finger is really higher. "We jterceive the contacts as 

 double because we refer them to two distinct parts of 

 space." The touched sides of the two fingers are normally 

 not together in space, and customarily never do touch one 

 thing ; the one thing which now touches them, therefore, 

 seems in two places, i.e. seems two things.* 



There is a whole batch of illusions which come from 

 optical sensations interpreted by us in accordance with our 

 usual rule, although they are now produced by an unusual 

 object. The stereoscope is an example. The eyes see a 

 picture apiece, and the two pictures are a little disparate, 

 the one seen by the right eye being a view of the object 

 taken from a point slightly to the right of that from which 

 the left eye's picture is taken. Pictures thrown on the two 

 eyes by solid objects present this identical disparity. 

 Whence we react on the sensation in our usual way, and 

 perceive a solid. If the pictures be exchanged we perceive 

 a hollow mould of the object, for a hollow mould would 

 cast just such disparate pictures as these. Wheatstone's 

 instrument, the pseudoscope, allows us to look at solid 

 objects and see with each eye the other eye's picture. We 

 then perceive the solid object hollow, if it he an object ivhich 

 migJit probably be hoUoic, but not otherwise. A human face, 

 e.g., never appears hollow to the jjseudoscope. In this 

 irregularity of reaction on difierent objects, some seem 

 hollow, others not ; the perceptive process is true to its 

 law, which is always to react on the sensation in a deter- 

 minate and figured fashion if possible, and in as probable 

 a fashion as tJie case admits. To cofiple faces and hollow 



* The converse illusion is bard 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 as a malter 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. AVhen the fingers are pressed together we feel one object to 

 be between them. And when the fingers are crossed, and their correspond- 

 ing points a and b simultaneously ^ress^tf, we do get soraothinjf like the 

 illusion of singleness — that is, we get a very doubtful doubleness. 



