Proximity

                          X                  X
                          X                  X

                          X  X
                          
                          
                          
                          
                          
                          
                          X  X

Similarity of size and color

                             X     O




                             X     O

                             X   X


                              O            O

Good figure

Not-such-a-good figure

Common fate

• 
• Coherence illusion applet: http://www.ed.uiuc.edu/courses/edpsy387/fa98/java-illusion/