Look through a series of vertical bars, as those of a palisade, or of
a Venetian window-blind, at the wheel of a carriage passing along the
street, and the spokes of the wheel, instead of appearing straight, as
they naturally would do, if no bars intervened, seem to be of a curved
form. The velocity of the wheel must not be so great as to prevent the
eye from following the spokes as they revolve.
Again, when the disk of the wheel, instead of being marked by a number
of radiant lines, has only one radius marked upon it, it presents the
appearance, when rolled behind the bars, of a number of radii, each
having the curvature corresponding to its situation, their number being
the same as that of the bars through which you look at the wheel. It
is, therefore, evident that the several portions of one and the same
line, seen through the intervals of the bars, form on the retina of the
eye so many different radii.