Bauhaus-Universität Weimar

Optical Illusions of Motions
Hall, Granville Stanley Henry P. Bowditch
objects, although those lying beyond the window-pane (and hence not 
accommodated for) are seen to flit past as before. Neither this nor the 
analogous experiment of Helmholtz upon the way in which opening 
and closing the eyes affects the apparent motion of objects after the 
cessation of a whirling movement of the body round its own axis 
is entirely in accordance with our observations. This hypothesis might 
explain the after-effects of any continuous or uniform motion in one 
direction, as, e.g., when the drum of an ordinary kymograph suddenly 
stops ; but the apparently opposite movement of a rotating disk when 
arrested, which Helmholtz thinks due to slight and probably circular 
movement of the eyeball following the movement of attention in the 
indirect field of vision, has led us to the opinion that modifications of 
muscular tension are but one of the less important factors in the 
explanation of this illusion. The phenomena of the rotating spiral 
which Helmholtz admits belongs to the same class of illusions, and 
especially the opposite after-effects of two spirals rotating in opposite 
directions at the same time and near together observed by us, seem 
conclusive against this theory. 
More recently Dr. F. Guthrie, Aitken1, and especially Professor 
SilvanusP. Thompson, have made valuable additions to our knowledge 
of these phenomena2. The apparent movement of the moon behind fast- 
flying clouds, the apparent backward motion of the water near the shores 
of a large river if the current is rapid at the middle, the apparent ducking 
forward of the masts of a ship in passing under a bridge from which it is 
observed, the seeming revolution of the landscape about any intermediate 
fixation point v-iewed from a moving train, the backward motion of 
stationary objects after viewing a procession,—these are a few of the 
more familiar forms of this class of illusions which Thompson proposes 
to call illusions of “ subjective complementary motion ” and to regard 
them as analogous to the more familiar phenomena of subjective com¬ 
plementary colours. His law is stated as follows : “ The retina ceases to 
perceive as a motion a steady succession of images that pass over a par¬ 
ticular region for a sufficient time to induce fatigue, and on a portion of 
the retina so affected the image of a body not in motion appears by 
contrast to be moving in a complementary direction.” But the analogy 
between colours and motions in different directions in space is at best 
only remote and symbolic, so that the explanatory power of this law is 
1 Proceedings of the Royal Society of Edinburgh^ov., 1878. 
2 See articles by S. P. Thompson in The Philosophical Magazine, 1876, in Quarterly 
Journal of Science, March, 1879, in Brain, October, 1880.


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