Our visual sensations enable us to perceive the existence, position and correct form of the various objects around us. For visual perception much more is necessary than the mere perfection of the dioptric media of the eye, and of the retinal nerve mechanisms. Besides the changes produced in the retina by light and the excitations in the nerve cells of the visual centre, there must be psychical action in other cells of the cortex of the brain. This psychical action of the brain consists of a series of conclusions drawn from the experiences gained by our visual and other sensations.
Our ideas of external objects are not in exact accord with the image produced on the retina and transmitted to the brain, but are the result of a kind of argument carried on unconsciously in our minds. Thus, when no light reaches the retina, we say (without what we call thought) that it is dark; our retina being unstimulated, no impulse is communicated, and the sensation of blackness arises in our sensorium. When luminous rays are reflected to the retina from various objects around us, the physiological impulse starts from the eye, but in the brain, by unconscious psychical activity, it is referred in our minds to the objects around us, so that mentally we project into the outer world what really occurs in the eye. So also, from habit, we re-invert in our minds the image which is thrown on the retina upside down, by the lens, and so unconscious are we of the psychical act that we find it hard to believe that our eyes really receive the image of everything inverted, and our minds have to reinstate it to the upright position.
One of the most important means employed to enable us to form accurate visual perceptions is the varied motion which the eyeballs are capable of performing.