A number of interesting perceptual phenomena occur when circles, spirals, ellipses, and overlapping geometric figures are rotated. "A rotating circle seems to stand still, a rotating spiral seems to expand or contract, and an ellipse displays flowing deformations when it is turned." These phenomena are explained in terms of "subjectively identical points along the contour of these figures that arise in relation to the absolute directions of visual space." If one views these deformations monocularly, "...a solid form or a rigid plane figure moving in depth may be seen.".