Curious _illusions of movement_ in objects occur whenever the eyeballs move without our intending it. We shall learn in the following chapter that the original visual feeling of movement is produced by any image passing over the retina. Originally, however, this sensation is definitely referred neither to the object nor to the eyes. Such definite reference grows up later, and obeys certain simple laws. We believe objects to move: 1) whenever we get the retinal movement-feeling, but think our eyes are still; and 2) whenever we think that our eyes move, but fail to get the retinal movement-feeling. We believe objects to be still, on the contrary, 1) whenever we get the retinal movement-feeling, but think our eyes are moving; and 2) whenever we neither think our eyes are moving, nor get the retinal movement-feeling. Thus the perception of the object's state of motion or rest depends on the notion we frame of our own eye's movement. Now many sorts of stimulation make our eyes move without our knowing it. If we look at a waterfall, river, railroad train, or any body which continuously passes in front of us in the same direction, it carries our eyes with it. This movement can be noticed in our eyes by a bystander. If the object keep passing towards our left, our eyes keep following whatever moving bit of it may have caught their attention at first, until that bit disappears from view. Then they jerk back to the right again, and catch a new bit, which again they follow to the left, and so on indefinitely. This gives them an oscillating demeanor, slow involuntary rotations leftward alternating with rapid voluntary jerks rightward. But _the oscillations continue_ for a while after the object has come to a standstill, or the eyes are carried to a new object, and this produces the illusion that things now move in the opposite direction. For we are unaware of the slow leftward automatic movements of our eyeballs, and think that the retinal movement-sensations thereby aroused must be due to a rightward motion of the object seen; whilst the rapid voluntary rightward movements of our eyeballs we interpret as attempts to pursue and catch again those parts of the object which have been slipping away to the left.