Shadows fade and are lost at long distances because the larger quantity of illuminated air which lies between the eye and the object seen tints the shadow with its own colour.
177.
a b will be darker than c d in proportion as c d is broader than a b.
[Footnote: In the original MS. the word lume (light) is written at the apex of the pyramid.]
178.
It can be proved why the shadow o p c h is darker in proportion as it is nearer to the line p h and is lighter in proportion as it is nearer to the line o c. Let the light a b, be a window, and let the dark wall in which this window is, be b s, that is, one of the sides of the wall.
Then we may say that the line p h is darker than any other part of the space o p c h, because this line faces the whole surface in shadow of [Footnote: In the original the diagram is placed between lines 27 and 28.] the wall b s. The line o c is lighter than the other part of this space o p c h, because this line faces the luminous space a b.
Where the shadow is larger, or smaller, or equal the body which casts it.
[First of the character of divided lights. [Footnote 14: lumi divisi. The text here breaks off abruptly.]