CHAP, vi.] SOME OTHER SENSATIONS. 1069 



changes in the nerves themselves, but they may arise through 

 changes in the central organs ; we may be subject to tactile 

 phantoms comparable to ocular phantoms. Compared with vis- 

 ual sensations however our tactile sensations are so to speak 

 fragmentary. A momentary exposure of the retina may fill the 

 .mind with a complex visual image, full of the most varied inci- 

 dent , but the tactile impressions which we can receive at any 

 one moment are few and simple. Hence our tactile phantoms 

 are also simple ; we may fancy that some invisible garment has 

 swept past us, or that a scorching flame has passed near us, we 

 may feel that the hand or that the head is swollen and large, and 

 we may experience an imaginary pain in every region of the 

 skin in turn ; but the most that we can thus feel is simple com- 

 pared with the possible complexity of an ocular or even an audi- 

 tory phantom. 



666. Like other sensations our tactile sensations while 

 they sometimes give us trustworthy information of the exter- 

 nal world at other times may give rise to illusions. This is 

 well illustrated by the so-called experiment of Aristotle. It 

 is impossible in an ordinary position of the fingers to bring the 

 radial side of the middle finger and the ulnar side of the ring 

 finger to bear at the same time on a small object such as a marble. 

 Hence when with the eyes shut we cross one finger over the 

 other, and place a marble between them so that it touches the 

 radial side of the one and the ulnar side of the other, we recog- 

 nize that the object is such as could not under ordinary condi- 

 tions be touched at the same time by these two portions of our 

 skin, and therefore judge that we are touching not one but two 

 marbles. Upon repetitidn however we are able to correct our 

 judgment and the illusion disappears. 



