Bauhaus-Universität Weimar

Again, let the area be filled entirely with black or with 
white whereby the hypothesis would require a like result 
for both figures. The result gave (constants as before 
except angle line 10 mm.), 9 mm. and 13 mm. respec¬ 
The hypothesis evidently falls completely. 
Another hypothesis asserts that the illusion is due to 
the over-estimation of acute angles. According to such 
a hypothesis the illusion would reach a maximum at 
30°, and would increase steadily with the length of the 
angle line ; this is refuted by the experiments. More¬ 
over, a direct experimental refutation can be made. 
Take the two Figs. 105 where there are no acute 
angles at all. The results gave 8 mm. and 10 mm. 
Still another hypothesis explains the illusion by in¬ 
voluntary, forced movements of the point of regard. In 
his experiments Heymans was accustomed to place the 
constant line to the left of the figure. Happening to 
turn the figure around, he noticed that the illusion which 
had previously decreased owing to practice, reappeared 
with fuller force than ever. He also noticed a surpris¬ 
ingly irresistible impulse to follow the angle lines, 
especially the middle ones. Such impulses are un¬ 
doubtedly present; how could they bring about the 
illusion ? 
The fact of a most favourable size for the angle and 
the angle lines seems to indicate that two forces are at 
work, one to assist the illusion, the other to suppress it.


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