Parts of the scene reflect or emit light in a certain pattern; this light travels to the eye by a more or less straight path, and is focused by the lens to form an image on the retina; the retinal cells are stimulated in proportion to the intensity and spectral distribution of the light that falls on them; these stimulated cells stimulate other cells in turn, and so on, and the stimulations comprise a signal which propagates up the optic nerve into the brain; and finally there is a pattern of stimulation in the brain cells which either is or else causes the subject's visual experience.