Optical illusions of motion / by G. Stanley Hall.

  • Bowditch, H. P. (Henry Pickering), 1840-1911.
Date:
[1882]
Catalogue details

Licence: Public Domain Mark

Credit: Optical illusions of motion / by G. Stanley Hall. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

    4/71 (page 300)
    nf +hP sDinil. We should expect that a straight line or a square H^t'Liro upon the disk so that each Hue passed across hung betoie ^^^^ ^^^.^^^^ revolution. W Sfetpt d^^^^^^^^^ ^vherethe iu.age of the line left We should e p .^^^^g^ ^^^^ ^^g^^ 'for 1^^^^^^^^^^^ area oftheLo coincided. ^^^^^^ he ^ In no case is such a line or square rn the least deflected, ^th ugh the illusions of contraction and expansion are unnnpan-ed. The effect is perhaps best observed by transfernng tbe gaze rom the effect P ^ of paper in the neighbourhood T ZlJlt^^^^ The'surface of the paper seems to be animated t^t^, or expanding movement, while the line does not it ?s d Ition in^the least. Even for that part of the retii^ ;vhere he 1 usion is greatest the ability to judge of the straaghtness of lines is not s n ibly alected. Even when lines pass across two spirals revolving • f opp sHe' directions, and giving rise to distinct and opposite after- effects he eye being fixed upon a point between them no deflexion lhateve could be observed. The after-image of tbe revolving spiral x Ibstwed w th closed eyes appears as a gray or yellow sh crrcular tract in which the natural granulation of the field is more distinct than else- where on account of a peculiar boiling or seething sensa ion, which it is lot easy to characterize further and from wMch alone i is not very easy te tell in whicb direction the spiral had revolved. This tract the hue crosses without bending, and if the line is -a broad one and - l-d across the disk just before it has stopped revolving the boihng sensation can be reahzed on or through the after-image of the former. ^ ^ Thus the same retinal elements stimulated at the same time give rise io the two opposite impressions of motion and rest. How this occurs we can only conjecture at present. Whatever may be the explanation of this phenomena the effect of the experiment is very striking, for the field of vision as a whole appears to be animated with a movement ot contraction or expansion while the separate points in the field do not change their relative positions. The readiest assumption seems to be that the impression of motion either affects a different cerebral centre than that of the rest, or that one is more centralized or perceptive and the other more peripheral or more purely sensory. It is impossible to conceive how this persistent after-impression of motion can be a product of experience or association^ Experience cannot overcome nor volition reverse it as it may certain perspective figures. We cannot resist I See Helmholtz, Physiohghchc OptU,; ]). 438.