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 color,' one in which the second color is in the
background and is viewed through an opening in the first. With such an
arrangement we find that we get the series of bands both when the wire
is passed in front of the disc and when passed in back between disc
and background; and further experimentation shows that the time