A Treatise on Painting, by Leonardo da Vinci
Senex and Taylor, London
There are several places really enlighten'd, which nevertheless appear full of darkness and obscurity, and where the things that are found, remain entirely devoid both of Form, and Colour. This Phænomenon is owing to the Light of the Air, interposing it self between the Eye and the Object; and appears very sensibly in Windows, which when view'd from a far, the Eye sees nothing within them, but a continued uniform obscurity; whereas entring into the places themselves, you will find them well illumined, insomuch that you may be able to distinguish the Figures and Colours of the most minute Objects within them. These two very different Impressions, are owing to the Natural Disposition of the Eye, whose weakness being unable to support the too powerful brightness of the Air, the Pupil contracts it self, and by that means loses a great deal of its force; on the contrary, in places more obscure, the Pupil dilates it self, and acquires new force, in proportion as it increases in extent; by this means, taking in the Images of Objects, and seeing their parts very distinctly, which before were invisible.