Rotating spiral (cone)
If you slowly rotate the disc, you can observe how the impression of a spatially rotating cone and funnel crater is produced. Even a change of angle by walking from side to side does not change this Impression.
Further informationson the exhibit "Rotating spiral (cone)"
The eye looks for a virtual center in the asymmetrical image. However, it succeeds only by creating a third dimension, i. e. perspective. Thus the cone or funnel appears three-dimensional. If you look at the disc with only one eye, the effect is intensified.