The theory of vision: light, colour and the eye
Coloured lanterns, the camera obscura and the perforated nerve of the eye
In this optical treatise Leonardo argues that objects fill the surrounding air with the 'species' of their form and power, using the example of a coloured lantern that tints everything it lights, of church windows, and of the red sunset that dyes the clouds. He then weighs the opposing view — that the eye, like the ear, only receives images — citing the pinhole in a shuttered window that projects inverted images of outside bodies, a camera obscura. Turning to the eye itself, he describes the ring of light around the black centre as a perforated nerve carrying the apprehensive power to the common sense. He compares the outer humour to hunting hounds that raise the game while the pupil, like greyhounds, seizes and imprisons the chosen images in the memory; a small eye diagram accompanies the text.
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Coloured light tints what it illuminates
A lamp placed in a lantern tinted green or another transparent colour makes everything it lights appear that colour; likewise church windows colour the light passing through them, and the setting sun, reddened by vapours, dyes red all the clouds it illuminates. Leonardo offers these as proof that bodies send out the species of their powers without harm to themselves.
The pinhole image (camera obscura)
Against the claim that the eye merely receives, like the ear, he cites a small aperture made in a window that renders inside the room all the similitudes of the bodies before it, in colour and figure but turned upside down. If the eye worked the same way, he objects, everything seen would appear inverted.
The eye: perforated nerve and pupil
The ring of light in the white of the eye is apprehensive of objects and holds a black point, a perforated nerve running to the inner faculties, full of the apprehensive and judging power that reaches the common sense. The surrounding humour acts like the hounds that raise the game, while the pupil, like greyhounds, seizes the pleasing images and locks them in the prison of memory.
Diagram of the eye with hunting labels a, b, o
A small figure of an eye is lettered a and b for the greyhounds (leverieri) and o for the hound (bracco), illustrating the metaphor by which the outer humour rouses the images while the central pupil captures them. The letters key the drawing to the hunting simile in the text.