There are many different ways of perceiving the world.  One of these ways is by perceptual organization.  Perceptual organization are processes used to group basic sensory elements together into perceivable objects.  The two basic principles of perceptual organization involves perceiving figures embedded in a perceptual background and organizing what you see, or what you perceive, in the simplest form possible.  Another type of perception is depth perception, which is primarily utilized by visual or auditory senses.  The organ used for visual perception is the eye.  The eye is an extremely complex organ.  It is composed of multiple of structures each having it's own specific function or functions (refer to diagram).  The cornea is a clear protective membrane which allows for the passage of light but blocks the entrance of dust.  The lens is just behind the cornea and it focuses the light entering into the eye.  The light forms an image which falls on the retina.  This image is upside down and is processed by the brain.  The amount of light entering the eye is controlled by the iris, the colored structure of the eye.  The pupil is a black dot in the center of the eye through which light passes.  The retina contains over 100 million nerve cells known as rods and cones.  Cones are responsible for colored vision and rods are used for black and white vision.