What is the ellipse in figure 1 more: the lower or inner top? It is difficult to avoid the thought that the lower the top. Meanwhile, both are equal, and only the presence of the outer, enclosing ellipse creates the illusion that it contained the ellipse is smaller than the bottom.
Figure 1. Which the ellipse is greater the lower or inner top?
The illusion is enhanced and the fact that the whole figure seems to us not flat, and bodily - in the form of buckets: ellipses unwittingly become us prospectively in tight circles, and lateral straight lines in the bucket.
Figure 2 the distance between the points and a and b seems to be more than between points m and n. The presence of a third straight line emanating from the same vertex, reinforces the illusion.
Figure 2. What distance is more AB or mn?