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Explanation of an optical deception in the appearance of the spokes of a wheel seen through vertical apertures

1833; Royal Society; Volume: 2; Linguagem: Inglês

10.1098/rspl.1815.0252

2053-9142

Peter Mark Roget,

Historical Geography and Cartography

The optical deception which Dr. Roget describes, takes place when a carriage-wheel, rolling along the ground, is viewed through the intervals of a series of vertical bars, such as those of a palisade, or of a Venetian window-blind. In these circumstances, all the spokes ap­pear curved, those which are situated vertically excepted, the degree of curvature increasing as their position recedes from the vertical, and being a maximum for the horizontal spokes. The convexity of these curved images is always turned downwards, and the direction of their curvature is the same whichever way the wheel be moving. To determine the influence of variations of circumstances on these phenomena, appeared to Dr. Roget the proper mode of obtaining a clue to their explanation. It results from his experiments, that a certain degree of velocity in the wheel is necessary to produce the deception in question; and if this be communicated to it gradually, the principal effect of curvature is observed to come on suddenly, but the degree of bending is independent of the velocity of the wheel, and each image appears, during the moment it is viewed, motionless. The number of spokes makes no difference in their degree of curvature. The deception is favoured by the diminution of the interval between the bars, by throwing a strong light on the wheel, and by every circumstance which tends to draw attention from the bars and fix it on the wheel. The number of curved images was found to depend on the ratio of the angles subtended at the eye by the intervals between the bars, and those between the extremities of the spokes; being greater as this ratio was less. If the bars were inclined to the horizon, the phenomena were those of a wheel revolving with a less velocity in a direction perpendicular to their length. Finally, the combination of a progressive with a rotatory motion (whether produced by a real motion of the wheel, the bars, or the spectator,) was found to be essential to the production of the phenomena.