( ^» ) 



It Is well known that two areas of different brightness, either 

 adjoining' each other, or at some distance of each other standing out 

 against the same background, when viewed in one glance, influence 

 each other's apparent brightness by contrast, so that the brighter area 

 causes the brightness of the darker area apparently to decrease and 

 conversely. Several psychologists Lehmann '), Ebbinghaus '^), Hess 

 and Pketori ^), Kirschmann *) and others have made this phenomenon 

 a subject of investigation and have brought to light some regularities 

 in its occurring; the laws that govern this phenomenon have however 

 not yet become completely known in spite of the important inve- 

 stigations of these scientists. 



Now we should be inclined to suppose that the maxima and 

 minima of brightness described above are to be reduced to the 

 said effects of contrast. But then it would seem strange that 

 there should be no intensifying whatever of the lines, whenever the 

 transitionzone disappears between two areas of equal bright- 

 ness ; for we might expect that especially in this case there would 

 be a prominent maximum on the one side of the border and a pro- 

 minent minimum on the other side of it. But as it is, each of the 

 two zones shows almost uniform brightness (fig. la, which shows 

 the disc of fig. lb while rotating) although it is not to be denied, 

 that in both areas there can be detected, towards the limits a slight 

 variation of brightness in the sense to be expected. Similar changes 

 appear a little more distinct in a zone of uniform brightness bounded 

 on the one side by a brighter zone, on the other side by a darker 

 zone as represented in fig. 8a (corresponding to fig. 8i); but although 

 in this case the contrasting influence of the neighbouring zones is 

 more visible, yet the phenomenon observed shows quite another 

 character than in the case of the presence of a transitionzone. 



5. Presently we shall say a few words more about a possible 

 explanation of the optical illusion ; first however I'll give some 

 further information about the conditions under which it appears and 

 about the laws which seem to govern it : 



P. In the case of a transitionzone with constant gradient of 

 decreasing or iucieasing intensity the maximum and minimum appear 

 exactly on the borders of the transitionzone ; at any rate the devia- 



1) Lkhmann, Wundt's Philosophische Studiën 3, S. 497, 1S86. 



2) Ebbinghaus, Berl. Sitzber. 1887, !S. 9!I5. 



') Hess a. Pretori, Von Graefe's j^rebiv f. 0|)litlialmologie 40, 4. 



■*) Kirschmann, Wundt's Ptiylosopliische Studiën 6, S. 417, 1891. 



