Why two horses on parallel courses seem to run at each other
Their images move toward the pupil, the surface centre of sight
Leonardo argues that if the eye stands midway between two horses running to a common finish along parallel tracks, the two will appear to run one against the other rather than side by side. He explains the illusion by the way the images (simulacra) of the horses impress themselves on the eye and travel toward its surface centre, the pupil. The single ink diagram places an eye at the left with rays fanning out to points strung along an upper and a lower horizontal line, the two parallel courses.
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Two horses on parallel courses appear to converge
When the eye is set in the middle of the run of two horses that race toward a shared finish along parallel courses, they will seem to run one against the other. The parallel tracks are read by the eye as lines closing on each other.
Images gather toward the pupil, the surface centre of the eye
The apparent convergence happens because the images of the horses that are impressed upon the eye move toward the surface centre, which Leonardo names as the pupil. Perceived direction of motion is thus a fact about where the image lands on the eye, not about the true courses outside.
Ray diagram from a single eye to two parallel lines
A sketched eye at the left sends straight rays out to a row of small circles on an upper horizontal line and to another row on a lower horizontal line. The construction shows the two parallel courses and the sightlines that gather back to the eye.