The Da Vinci Notebooks at sacred-texts.com
The relations of luminous to illuminated bodies.The middle of the light and shade on an object in light and shade is opposite to the middle of the primary light. All light and shadow expresses itself in pyramidal lines. The middle of the shadow on any object must necessarily be opposite the middle of its light, with a direct line passing through the centre of the body. The middle of the light will be at a, that of the shadow at b. [Again, in bodies shown in light and shade the middle of each must coincide with the centre of the body, and a straight line will pass through both and through that centre.]
81:58 : In the original MS., at the spot marked a of the first diagram Leonardo wrote primitiuo, and at the spot marked c--primitiva (primary); at the spot marked b he wrote dirivatiuo and at d deriuatiua (derived).