Molecular model. Computer model of the molecular structure of a crystal, used to demonstrate ideas on perspective. This is a regular array (crystal) of atoms (yellow). The views through the crystal are often blocked, but sometimes an unobstructed view is seen between rows of atoms. This is called the orchard effect. Important aspects of number theory, such as the distinction between rational and irrational numbers, emerges from such models. This model was created as an artwork by Professor Eric Heller.