Principles of Animation - Animation is possible because of a biological phenomenon known as persistence of vision and a psychological phenomenon called phi.An object seen by the human eye remains chemically mapped on the eye's retina for a brief time after viewing.Combined with the human mind's need to conceptually complete a perceived action,this makes it possible for a series of images that are changed very slightly and very rapidly,one after the other,to seemingly blend togather into a visual illution of movement.The follwing shows a few cells or frames of a roting logo.When the images are progressively and rapidly changed,the arrow of the compass is preceived to be spinning.