Bauhaus-Universität Weimar

Optical Illusions of Motions
Hall, Granville Stanley Henry P. Bowditch
u rinsing table was at the point of its lowest depression moving, e.g., 
to the, right, and this retinal field would be confused by movements 
in no other direction. The movement of a straight line in the direction 
of its length, however, is not calculated to give a vivid motor effect. 
But when, after half a rotation the cogs, which project from the upper 
surface of the horizontal line, appear in a clear field moving off to the 
left, the motor effect, oft repeated, is vivid, for the points attract 
attention and concentrate and intensify the impression of motion. The 
backward motion of the cogs to their old position while the table is 
depressed is less distinctly perceived or realized, partly because they 
move through a portion of the field of vision partially obscured by after* 
images of other parts of the figure, and partly because the attention is 
constantly more occupied with those parts of the aggregate phenomenon 
not thus obscured, but where motor effects are more distinguishable 
and striking. This series of processes described for the lower line with 
upward cogs would, in Fig. 9, alternate with a corresponding series ot 
movements in the opposite direction in the upper line with downward 
If these horizontal lines were bent into semicircles, we should have, of 
course, the case of the wheel with inward cogs (Fig. 8a). Here we 
must hold fast to the same distinction between the area of confused after¬ 
images moving in different directions where specific effects are lost to 
consciousness, and the peripheral areas, through which the movement of 
projecting parts is always in the same sense, and to which attention is 
chiefly attracted, as we have pointed out above. Here too it is not easy 
to measure, even approximately, the rate of motion so purely illusory, 
and the inference of which is from so extremely complex sensuous 
impressions as in this case. The degree of illusion does not seem to be 
strictly proportionate to the rapidity of the actual motion. If it is slow, 
and the attention is concentrated upon a single point or cog, the illusiou 
often vanishes from that point, while the points indirectly seen still seem 
to move despite the evident impossibility of such movement, without 
distortion of the figure. If the motion is rapid, so that there is no fixation 
point, and the attention is diffused over the whole figure, the innermost 
edge of the aggregate image often seems illusively to move more rapidly 
than the peripheral half, and independently of it. The general effect of 
motion is best secured in indirect vision, and sometimes is aided by 
improper accommodation. This form of the circle with inward cogs 
affords the most striking of all these illusions. The deception is more 
complete than in the previous case of parallel cogged lines, because the


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