Optics

Overview

Leonardo's optics work is inseparable from his painting. He needed to understand how the eye sees in order to master what it sees. The result is a remarkable body of investigation into the mechanics of vision: how the pupil contracts and dilates, how images are inverted and transmitted to the brain, why we see in perspective, and how mirrors and lenses work.

His most important optical discovery was the camera obscura principle — that images passing through a small hole into a dark room are projected inverted on the opposite wall. He described this in precise detail and connected it to the functioning of the human eye. He also conducted careful experiments on binocular vision, pupil dilation in response to light, and the behavior of nocturnal animals' eyes.

Most of the optics material comes from MS. D at the Institut de France, a small notebook almost entirely devoted to the eye and vision.

Nature has not made a uniform power in the visual faculty but has given this faculty greater power in proportion as it is nearer to its centre, and this it has done in order not to break the law given to all other powers which have more potency in proportion as they approach nearer to this centre.

And this is seen in the act of the percussion of any body, and in the supports of the arms of the balance where the gravity of the weight is lessened as it draws nearer; it is seen in the case of columns walls and pillars; it is seen with heat and in all the other natural powers.

MS. D, i r.

Nature has made the surface of the pupil situated in the eye convex in form so that the surrounding objects may imprint their images at greater angles than could happen if the eye were flat.

MS. D, i r.

The pupil of the eye changes to as many different sizes as there are differences in the degrees of brightness and obscurity of the objects which present themselves before it.

In this case nature has provided for the visual faculty when it has been irritated by excessive light by contracting the pupil of the eye, and by enlarging this pupil after the manner of the mouth of a purse when it has had to endure varying degrees of darkness. And here nature works as one who having too much light in his habitation blocks up the window half-way or more or less according to the necessity, and who when the night comes throws open the whole of this window in order to see better within this habitation.

Nature is here establishing a continual equilibrium, perpetually adjusting and equalising by making the pupil dilate or contract in proportion to the aforesaid obscurity or brightness which continually presents itself before it.

MS. D, 5 v.

The images of the objects in the eye when making their entry into the eye deflect their rays in a way that is proved in perspective when these images pass from the density of the water to the thinness of the air.

MS. D, 7 v.

Experience which shows that the objects transmit their images or likenesses intersected within the eye in the albugineous humour shows what happens when the images of the illuminated objects penetrate through some small round hole into a very dark habitation. You will then receive these images on a sheet of white paper placed inside this habitation somewhat near to this small hole, and you will see all the aforesaid objects on this paper with their true shapes and colours, but they will be less, and they will be upside down because of the said intersection.

MS. D, 8 r.

The image of every object is changed in the mirror so that its right side is opposite to the left of the object reflected and similarly the left to the right. This is of necessity the case because every natural action is performed by nature in the shortest manner and the briefest time possible.

MS. D, 4 r.