The document discusses the anatomy and physiology of human vision. It describes the main components of the eye including the cornea, iris, pupil, lens, retina and optic nerve. Light enters through the cornea and pupil, is focused by the lens and forms an image on the retina. The retina then sends this visual information to the brain via the optic nerve to be interpreted. For virtual worlds to mimic real worlds, they must properly stimulate the visual system by matching properties like size, shape, color, brightness, contrast, focus and depth. The more a virtual world simulates these visual components, the more immersive and realistic it will appear.