A.S. Luchins & E.H. Luchins: Comments on the Concept of Closure
   Gestalt Archive: Comments on the Concept of Closure by Abraham S. Luchins and Edith H. Luchins
   http://www.gestalttheory.net/archive/closure.html

On Perceptual Grouping
   Adapted from Max Wertheimer, Untersuchungen zur Lehre von der Gestalt, II. Psychologische Forschung, 1923, 4,301-305. By Charles F. Schmidt
   http://www-rci.rutgers.edu/~cfs/305_html/Gestalt/wertheimer2.html

Rudolf Arnheim: Entropy and Art (1971)
   An Essay on Gestalt psychological views on disorder and order.
   http://acnet.pratt.edu/~arch543p/readings/Arnheim.html