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)

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    were screwed, one to each edge of a piece of thin board, 13, eight inches square, which could thus be made to move in circles of varying dimensions corresponding to the excentricity of the pivots. Upon this board were fastened the cards displaying the figures which produced, when rotated as above described, the illusions of motion which were to bo studied. Another board, E, of the same size, sliding in horizontal bearings, G G, could be connected by a crank movement with one of the pivots and thus used to impart to cards secured upon it a rapid horizontal movement to and fro. , When a chart upon which concentric circles are inscribed is rapidly shaken horizontally upon the vibrating table last described, the lines of the two opposite perpendicular sectors remain very distinct, while the lines upon the lateral sectors are blurred, or become quite gray if the movement is through a space equal to or greater than the width of the lines and is very rapid. This is due to the fact that so long as the movement is in the same direction as the white lines, as in the case of the perpendicular sectors, the same retinal elements are constantly stimulated, while if the movement is across them, as it is over the lateral sectors, alternating images of the dark and light lines follow each other in rapid succession across the same retinal surface. This fact is the key to the explanation of the phenomena caused by the same chart upon the other or rotating table when the circles seem to revolve. During every instant of the rinsing movement here, all lines in the same direction as the motion are distinctly seen, and all those at right angles to it are blurred. This explains the phenomena of alternate light and dark rotating sectors, and, in a large figure with many concentric circles the radius of most of which is very large compared with the excentricity of adjustment of the table, this is the chief cause of the illusion, which is distinctly observable if a dozen central circles are covered and only a few peripheral ones seen. At every instant all retinal impressions along one diameter are prolonged and intensified, while along another diameter at right angles to the first all impressions are shortened and obliterated, and the diameter on which these efiects are observed, being shifted at each instant by the rinsing movement of the chart, produces the effect of rotating sectors. But now it is plain that somewhere, near the centre of the entire figure of concentric circles, must be a circle some point in the circum- ference of which will always be presented to a fixed eye, because its radius is equal to that of the rinsing movement. This circle will, therefore, present the appearance of a circle revolving about a point in