Foreshortened shadows and why a flat surface looks larger
A red-hot iron bar looks thicker; a square body's shadow in foreshortening
The folio opens with apparent magnitude: among bodies equal in size, whiteness, background and length, the one with the flatter surface looks the larger, proved by an iron bar of even thickness whose red-hot middle appears fatter than the rest. It then asks what shadow a square opaque body casts from a spherical light, warning that in foreshortening neither the quantity nor the quality of shadows shows its true figure. A further construction (a d, b c) shows a shadow-edge made visible by foreshortening and grades a wall's brightness by the size of the luminous angle; captioned figures of a circle and of crossed rays also appear.
On this page
Why a flatter surface appears larger
Among bodies equal in size, whiteness, background and length, the one with the flatter surface appears the larger figure. A bar of iron of even thickness, made red-hot at its middle, proves it, for the glowing part looks thicker than the rest.
What shadow a square body casts from a round light
Leonardo asks what shadow a square opaque body will make when lit by a spherical luminous body. He warns that not only the quantity but the quality of shadows and their edges, seen in foreshortening, will not display their true figure.
A shadow-edge made visible by foreshortening
The boundary b c, invisible through the loss of the shadows, nonetheless appears visible by reason of foreshortening, just as a b and d c appear. The wall is brightest where a larger luminous angle strikes it and keeps less of the light's proper quality where a larger shadow-angle darkens it.