Harvard Psychological Studies, Volume 1 eBook

This eBook from the Gutenberg Project consists of approximately 678 pages of information about Harvard Psychological Studies, Volume 1.

This phenomenon was first observed by Muensterberg, and by him shown to Jastrow,[1] who, with Moorehouse, has printed a study, without, however, offering an adequate explanation of it.

   [1] Jastrow, J., and Moorehouse, G.W.:  ’A Novel Optical
   Illusion,’ Amer.  Jour. of Psychology, 1891, IV., p. 201.

I. APPARATUS FOR PRODUCING THE ILLUSION.

Any form of color-wheel may be used, but preferably one which is driven by electricity or clock-work, so that a fairly constant speed is assured.  Several pairs of paper discs are needed, of the ordinary interpenetrating kind which permit a ready readjustment of the ratios between the two sectors, as follows:  one pair consisting of a white and a black disc, one of a light-and a dark-colored disc (light green and dark red have been found admirably suited to the purpose), and a pair of discs distinctly different in color, but equal in luminosity.

The rod should be black and not more than a quarter of an inch broad.  It may be passed before the rotating disc by hand.  For the sake of more careful study, however, the rod should be moved at a constant rate by some mechanical device, such as the pendulum and works of a Maelzel metronome removed from their case.  The pendulum is fixed just in front of the color-disc.  A further commendable simplification of the conditions consists in arranging the pendulum and disc to move concentrically, and attaching to the pendulum an isosceles-triangular shield, so cut that it forms a true radial sector of the disc behind it.  All the colored bands of the illusion then appear as radial sectors.  The radial shields should be made in several sizes (from 3 to 50 degrees of arc) in black, but the smallest size should also be prepared in colors matching the several discs.  Such a disposition, then, presents a disc of fused color, rotating at a uniform rate, and in front of this a radial sector oscillating from side to side concentrically with the disc, and likewise at a uniform rate.  Several variations of this apparatus will be described as the need and purpose of them become clear.

II.  PREVIOUS DISCUSSION OF THE ILLUSION.

Although Jastrow and Moorehouse (op. cit.) have published a somewhat detailed study of these illusion-bands, and cleared up certain points, they have not explained them.  Indeed, no explanation of the bands has as yet been given.  The authors mentioned (ibid., p. 204) write of producing the illusion by another method.  “This consists in sliding two half discs of the same color over one another leaving an open sector of any desired size up to 180 degrees and rotating this against a background of a markedly different color, in other words we substitute for the disc composed of a large amount of one color, which for brevity we may call the ‘majority color,’ and a small amount of another, the ‘minority

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Harvard Psychological Studies, Volume 1 from Project Gutenberg. Public domain.
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