A new perspective: Leonardo’s view on optics

Here Leonardo studies how the structure and movement of the eye affects vision, and thus how vision is relative and variable.

For example, when the eye is pushed upwards with a finger, its rear half lowers and the imprensive, (Leonardo’s anatomical term for the receptive part that occupies the rear part of the eye), will see the objects as lowered.

Here, Leonardo observes that the flame of the candle has an oblong shape, but the eye surrounds it with a roundness because the light enters the eye through a circular opening.

This sheet looks at the anatomy and vision of a bird's eye. Leonardo studies and draws the function of what we now call the nictitating membrane or third eyelid of a bird, a transparent membrane that is drawn across the eye.

Leonardo also describes another movable membrane or opaque “cover”. He then analyses the purpose of these membranes and their movements: the protection of the eye in the fast flight (the transparent membrane) and the maintenance of the upward vision to spot predatory birds.

Given Leonardo's obsession with the study of water and light, it is not surprising that he was among the few to consider the problem of how water interferes with the visual perception of an object immersed in it.

In this first drawing, the eye, located above the surface of the water, sees the object half-immersed in the water in a distorted way...

...unlike the eye at water level in this drawing.

Leonardo carefully studied the relationship between eye, light and object, using the terrestrial observer, moon and sun as his models.

In this example the eye does not see the illuminated part of the object, which is possible only when the eye “sees all the original light”, that is its light source.

Two candles, one with a lower flame (top), the other higher (bottom), show how the shadows cast by an illuminated opaque body are much longer as the light that illuminates it is lower.

The blue colour of air is fully appreciable, according to Leonardo, at noon, in the absence of clouds or excessive humidity. He explains the blue of the air as an optical phenomenon, a consequence of the light reflected by the minute particles of air humidity on the dark background of the sky with the sphere of fire.

The drawing here represents the opposite situation, namely the sun (on the right) that radiates light partly intercepted by clouds (on the left).

Leonardo studies the relationship between shadows and lights in trees. He considers the shaded areas inside the tree, and the shadow cast by the tree on the ground.

The drawings relate to this last phenomenon: the sun, on the right, projects the shadow of the tree on the ground and the length of the projected shadow varies according to the inclination of the ground, represented at its greatest in the bottom drawing.

According to medieval and Renaissance ideas of perspective, the images of objects reached the eye in the form of a perfect pyramid, with its base in the object and vertex or point in the eye...

... In this sheet Leonardo doubts that the lines that form the pyramid are straight.

In this example Leonardo studies the reflection of light in a concave mirror, which he makes plans to build.

The purpose of this mirror has not been clarified with certainty; perhaps Leonardo designed it as an experimental device for optical studies, or to harness sunlight as a source of energy.