According to Leonardo, the most important of the five senses was sight, the eye being the “window of the soul”. Leonardo thought a great deal about how the eye functioned and how we experience the world in terms of light and shade. His studies of the effects of light on form provided the basis for a revolution in the way that light, colour and space were described in painting.
Based on the principle that all things in nature are governed by mathematics, Leonardo worked out systems for grading for the action of light and shadow. These needed to take into account percussion and movement, the dynamic forces of nature.
On this sheet, the varying degrees of light intensity on a human face are analysed according to the angle of impact. Leonardo explains that the parts of the face that receive the rays of light at a perpendicular angle will be the most brightly lit - points F, G and H. Points B, C and D on the other hand receive less light due to the oblique angle at which the light strikes them. Leonardo points out that the system also indicates the parts “deprived of light” at M and K.
On the basis that light “acts like a blow”, the analogy of a ball thrown against a wall is used to illustrate the principle. A ball thrown straight at a wall at a perpendicular angle will be “more effective” than one thrown at an angle by someone positioned at one of the extremities of the wall.
In Leonardo's words
Shadow shares the nature of universal things, which are all more powerful at their beginning and become infeebled towards their end…as the oak tree which is more powerful at the point of its emergence from the ground, that is to say where it is thickest.