( 482 ) 



Gad and Goldscheider have made methodical experiments con- 

 cerning the phenomenon, causing them to consider it as a sensation 

 brought about by summation of stimuli. 



Thej also have pointed out already the analogy with the after- 

 images, of the retina, produced by optic stimuli of short duration. 



An exact analysis however of the subjective phenomena, following 

 the experiment indicated b}' Goldscheider shows, that the anology 

 betw^een what happens for the skin and for the eye, is a far greater 

 one than w^e might think at first sight. 



For by this analysis -it becomes evident that something may be 

 added still to Goldscheider's description. The subjective impression 

 does not stop at the secondary seiisation, but constantly a tertiary 

 sensation may be observed. 



x\s soon as I am trying the experiment of Goldscheider upon 

 myself, applying on the skin with Ihe point of a blunt pin a prick 

 of very short duration, but very feeble, the skin being depressed 

 hardh" more than Vs millimeter, the first, primary sensation arises 

 almost simultaneously with tlie contact. After an interval of from 

 0.8 — 0.96 second the secondary sensation commences in accordance 

 with the description of Goldscheider. 



About from one to three seconds after this, a tertuiry sensation 

 folloivs, consisting in a peculiar feeling of irritation and itching, 

 compelling us involuntarily to rub the irritated portion of the skin 

 with the hand. This last sensation rises very slowly, reaching 

 its maximum in little more than one second, and disappearing again 

 with far greater slow^ness. The duration of the first sensation amounts 

 to a very small part of 1 second, the second sensation is like- 

 wise a short one, though generall}- souiewhat longer than the first; 

 the duration of the tertiary sensation is still longer, amountiiig to 

 from 2 — 10 seconds, and even nioi'e if the intensity of the pi'ick 

 was somewhat stronger. The curve of the three sensations may he 

 represented graphically by a figure. 



Figure 1. 

 The analogy with the visual after-images, such as these have been 

 described recently by Kries, Hess, Hamaker and others, is most 



