Where Light Falls Strongest: The Angle of Incident Rays
A ball thrown at a wall shows how rays striking at equal angles give the brightest light
Leonardo argues that a ray (or blow) striking between equal angles has the greatest power, and grows weaker as the angles become more unequal, illustrating the point with a ball thrown squarely versus obliquely at a wall. Applied to light, the brightest particle of an illuminated surface is where the radiant line falls between two equal angles (lines a g, a h, l a), while it dims where the incident line strikes at more dissimilar angles (b c d), and the darkest parts (m k) can be recognized the same way. Faint radiating construction lines are visible in the right margin. The page closes with the opening of a new chapter on how a body's shaded and lit boundaries shift according to the colour and light of whatever borders its surface.
On this page
The ball-against-a-wall analogy for angle of impact
A blow falling between equal angles is in the first degree of power; between unequal angles it is weaker in proportion as the angles are more misshapen. Thrown squarely at a wall, a ball strikes at equal angles; thrown while standing at one edge of the wall it strikes at unequal angles and the blow does not take hold.
Brightest where rays meet equal angles (a g, a h, l a)
Every terminated light seems to arise from a single point, so the illuminated surface is most luminous where the radiant line falls between two equal angles, shown at a g, a h and l a. It is least luminous where the incident line strikes between more dissimilar angles, as at b c d, and by this means the parts wholly deprived of light, as at m k, can also be recognized.
Boundaries changed by the neighbouring colour and light
A new chapter opens on how bodies accompanied by shadow and light always vary their boundaries according to the colour and light of the thing that borders their surface. It begins with the case of a body whose illuminated part meets its ground.