| W + p | = | W + s | , |
| r | r' |
r'(W + p) = r(W + s) ,
W (r' - r) = rs - pr' ,
and, again,
| W = | rs - pr' |
| r' - r |
Before asking if this pure-color band W can be identified with the bands observed in the illusion, we have to remember that the value which we have found for W is true only if disc and pendulum are moving in the same direction; whereas the illusion-bands are observed indifferently as disc and pendulum move in the same or in opposite directions. Nor is any difference in their width easily observable in the two cases, although it is to be borne in mind that there may be a difference too small to be noticed unless some measuring device is used.
From Fig. 4 we can find the width of a pure-color band (W) when pendulum and disc move in opposite directions. The letters are used as in the preceding case, and W will include no transition-band.
Fig. 4.
We have