i CUTANEOUS SENSIBILITY 47 



conscious judgments associated with our sensations. It, is thus 

 evident, that the exe.entric projection and ohjectilicat ion of our 

 sensations into the external world occur only according to the law 

 of experience. Aristotle's illusion has recently heen explained hy 

 Menderer and Ponzo in the above manner, but hy a more suhtle 

 analysis. 



They recognise the same cause for Aristotle's illusion, and use 

 it to explain a number of other illusions. Among the latter is 

 the converse observation discovered by Rivers, that on touching 

 with two rods the edges of the lingers which face in opposite; 

 directions when they are crossed, there is an impression of only 

 one rod between them. The same illusion is obtained on putting 

 two objects under the tips of the crossed fingers, which are then 

 confounded into one (Ponzo). 



Other similar illusions have been observed and studied by 

 Pouzo, in which, on displacing some part of the body, e.g. the lobe 

 of the ear, from its normal position, the impressions are still 

 referred to the region in space in which the displaced part is 

 normally situated. Along with these we must group the so- 

 called finger-exchange (Henri, Ponzo), in which when the lingers 

 are crossed stimuli acting on one finger are referred to another. 



The power of localisation at the cutaneous periphery is not 

 confined exclusively to sensations of contact, but extends to all 

 the cutaneous sensations. Ponzo has recently estimated errors 

 in the localisation of tactile and pain sensations by pricking 

 different parts of the body so as to stimulate single specific points. 

 He found that the magnitude of the error varies with the region 

 of the body. The maximal delicacy of localisation is found in 

 the tip of the tongue, end of the forefinger, and middle part of 

 the free border of the lower lip; the minimal in the lateral 

 surfaces of the thorax. Sensations of cutaneous pain may be 

 localised as exactly as tactile sensations (Ponzo) ; it is consequently 

 a fallacy to suppose that pain sensations cannot lie localised, or 

 are so no less exactly than tactile sensations. Thermal sensations, 

 too, are localised, but less accurately. A systematic study of this 

 subject is wanting, but the experiments of Rauber (1869), of 

 Goldscheider (1887), and the more recent work of Ponzo, show 

 that cold spots are capable of more exact localisation than heat 

 spots. The stimulation of two cold spots can be plainly appreciated 

 at a distance of 0'8-3 mm., while in the same region that of two 

 heat spots can be recognised separately only when 2-5 mm. 

 apart. 



Lastly, the power of distinguishing two punctifnrm contacts on 

 the skin must not be confused with the power of localising them; 

 similarly, two points viewed through a prism may he distinct 

 from one another, and at the same time be localised in a position 

 in space other than their real place. Thus, in observations made 



