Now a certain number of these stimulations which immediately precede M will determine the characteristic effect, the fusion color, for the point A' at the moment M. We do not know the number of unit-stimulations which contribute to this characteristic effect, nor do we need to, but it will be a constant, and can be represented by a distance x = A'A above the line A'C'. Then A'A will represent the total stimulus which determines the characteristic effect at A'. Stimuli earlier than A are no longer represented in the after-image. AC is parallel to A'C', and the characteristic effect for any point is found by drawing the perpendicular at that point between the two lines A'C and AC.
Just as the movement of the disc, so can that of the concealing pendulum be represented. The only difference is that the pendulum is narrower, and moves more slowly. The slower rate is represented by a steeper locus-band, PP', than those of the swifter sectors.
We are now able to consider geometrically deduced bands as 'characteristic effects,' and we have a graphic representation of the color-deduction determined by the interception of the pendulum. The deduction-value of the pendulum is the distance (xy) which it intercepts on a line drawn perpendicular to A'C'.
Lines drawn perpendicular to A'C' through the points of intersection of the locus-band of the pendulum with those of the sectors will give a 'plot' on A'C' of the deduction-bands. Thus from 1 to 2 the deduction is red and the band green; from 2 to 3 the deduction is decreasingly red and increasingly green, a transition-band; from 3 to 4 the deduction is green and the band red; and so forth.
We are now prepared to continue our identification of these geometrical interception-bands with the bands observed in the illusion. It is to be noted in passing that this graphic representation of the interception-bands as characteristic effects (Fig. 7) is in every way consistent with the previous equational treatment of the same bands. A little consideration of the figure will show that variations of the widths and rates of sectors and pendulum will modify the widths of the bands exactly as has been shown in the equations.
The observation next at hand ([p. 174], No. 5) is that "The total number of bands seen at any one time is approximately constant, howsoever the widths of the sectors and the width and rate of the rod may vary. But the number of bands is inversely proportional (Jastrow and Moorehouse) to the time of rotation of the disc; that is, the faster the disc, the more bands."
Psychological Review. Monograph Supplement, 17. Plate V.
Fig. 7.
