Leonardo wanted to understand the human eye, but its dissection was difficult. When cut into, the eye collapses into a gelatinous mess and the lens takes on a spherical shape. Leonardo tried boiling his specimens in egg white and water first, but this distorted the shape of the lens and detached it from the retina.

Instead he attempted to settle the question of the eye’s optics by deductive means, by drawing diagrams and applying logic to the problem. While this resulted in many correct observations, it also led to misleading conclusions on Leonardo’s part.

On Manuscript D Fol.4r in a ground-breaking study of the variable diameter of the pupil, Leonardo came to the odd conclusion that the larger the pupil the larger the appearance of the object it sees. He correctly observed however, that the eye sees and knows objects of vision with greater intensity when the pupil is more dilated. This was proved in the case of nocturnal animals such as cats and owls and other such animals “in which the pupil undergoes a great variation from large to small in the dark and in the light”.

In Leonardo's words
O marvellous necessity…O mighty process. Here the figures, here the colours, here all the images of the parts of the universe are reduced to a point; and what point is so marvellous?…These are miracles…forms already lost mingled together in so small a space, it can recreate and reconstitute by its dilation.