A moving eye fixed on a star sees other objects flee backward
Objects on the non-central lines appear swift and move contrary to the eye
Leonardo treats the case of an eye that shifts its position bodily while keeping its gaze locked on a star. He argues that everything seen by the non-central sightlines will appear swift and will seem to flee in the direction contrary to the eye's own movement. Using the letters d (the star), b and a (the eye's two positions) and c (the object), he explains that the object's image, changed many times across the non-central lines, seems to have travelled from n to c against the eye's motion. The diagram is a triangle of sightlines with the star and object at its far corners.
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Fixing on a star makes side objects flee contrary to the eye
If the eye moves from its place but holds its vision fixed on the star, every object caught by the non-central sightlines appears swift and seems to flee in motion contrary to the eye. Only the fixated point stays steady while its surroundings stream backward.
Eye b to a, star d, object apparently from n to c
With the eye at b gazing at the star d and moving bodily from b to a, its non-central lines exchange the image (species) of the object c many times. The object therefore appears to have moved from n to c, in motion contrary to that of the eye.
The star as the fixed point of the sightline
The star d is the object on which the moving eye keeps its central sightline. Its remoteness is what makes the nearer objects seem to rush the opposite way as the eye travels.