Why lights seem to sprout rays: reflection on the edges of the eyelids
Notebook D: the halo around a light is a reflection in the lids, not rays from the body itself
Leonardo asks why the rays of luminous bodies seem to lengthen with the distance between the light and the eye, and whether such rays arise from the eye or the body. He argues that the rays that show a light's edges do not come from the body but from its image imprinted on the thicknesses of the eyelids of the beholder: a wide-open eye shows no such rays, and a light seen through a pinhole in paper appears rayless. Invoking that the angle of incidence equals the angle of reflection, he says a nearly-closed eye mirrors the light in the edges of its lids, so the pupil takes three images of one light, two on the lid edges and one in the pupil, which merge; raising or lowering the face predictably strips away the rays that the lids can no longer see. Small marginal diagrams show single eyes and pairs of eyes with a luminous body and its rays.
On this page
The halo is a reflection, not the body's own rays
The rays that show the edges of luminous bodies do not originate from those bodies but from their image imprinted on the thicknesses of the beholder's eyelids. A wide-open eye shows no such rays, and a star or light seen through a tiny hole pierced in paper always appears without rays.
The angle of incidence equals the angle of reflection
The true proof rests on the ninth proposition of perspective: the angle of incidence is always equal to the angle of reflection. So the seeming rays arise when the almost-closed eye looks through the narrow slit between its lids, whose edges mirror the light and reflect it to the pupil; where the light does not strike, no reflected ray, and none reaches the pupil.
Three images: two on the lid edges, one in the pupil
The pupil takes three images of one and the same light: two in the thicknesses of the eyelid edges and one in the pupil. Being very close together they seem continuous and joined. Raising the face loses the lower rays and lowering it loses the upper, because a lid that neither sees nor is seen by the light cannot receive its image.
Eyes that catch the upper or lower rays
The marginal diagrams distinguish an eye that sees the ray of the luminous body upward, a second that sees the rays downward, and a third that sees both the upper and lower rays, above and below the light.