Among the most common optical illusions we may cite those which consist of the apparent motion of external objects. When on a boat, for example, or in a carriage which is in motion, we seem to be at rest while the shore or the sides of the road seem to be in motion. We have no consciousness of the movement of external objects except by being ourselves at rest, and when the image of an object moves across the retina while the eye and the body are in repose, the object seems to change its position relative to us. Carried along by the boat or carriage, without our bodies taking any active part in the movement, we judge of the relative displacement instinctively, and from habit we refer to external objects the movements which we not ourselves feel.
Sometimes there is an apparent displacement of objects, although neither the objects nor the eyes are in motion, but in a normal condition it is always after a movement of the body that this phenomenon appears. As when the body is whirled round rapidly and then suddenly stopped, everything seems to turn in an inverse direction. It is probable that the illusion then depends on the impulse to movement in a certain direction imparted to the brain; in feet, if we stop after turning round, the sensation of turning persists for some moments, especially in the head; and if we refer it instinctively to external objects, it is in consequence both of the persistence of the previous sensation, and of the idea of our actual immobility. We turn still, just as after having laid down a burden we continue to feel its weight upon us.
Gratiolet ascribes the apparent motion of objects under these circumstances to insensible oscillations, which displace to a limited extent the ocular axes, but he does not indicate the cause of these oscillations.
The visual impressions are transmitted from the retina to the brain by means of the optic nerve, of which that membrane appears to be the expansion. The two optic nerves converge from the base of the orbit toward the centre of the base of the skull, where there is an interlacement of their fibres in such a manner, that a portion of the right nerve goes to the left side of the brain, and a part of the left nerve to the right side; this is called the chiasma, or commissure of the optic nerves. Physiological theories, which are no longer tenable, have been deduced from this crossing of the nerves, and nothing positive is yet known of the relation between this disposition and the visual function. Mechanical irritation of the nerve seems to develop luminous impressions as in the retina, but it causes no pain whatever.