Leonardo da Vinci

“ An object represented in white and black will display stronger relief than in any other way; hence I would remind you O Painter! to dress your figures in the lightest colours you can, since, if you put them in dark colours, they will be in too slight relief and inconspicuous from a distance. ”

“ The colour of derived shadows is always affected by that of the body towards which they are cast. To prove this: let an opaque body be placed between the plane _s c t d_ and the blue light _d e_ and the red light _a b_, then I say that _d e_, the blue light, will fall on the whole surface _s c t d_ excepting at _o p_ which is covered by the shadow of the body _q r_, as is shown by the straight lines _d q o e r p_. And the same occurs with the light _a b_ which falls on the whole surface _s c t d_ excepting at the spot obscured by the shadow _q r_ ”

“ The colours of the rainbow are not produced by the sun, for they occur in many ways without the sunshine; as may be seen by holding a glass of water up to the eye; when, in the glass--where there are those minute bubbles always seen in coarse glass--each bubble, even though the sun does not fall on it, will produce on one side all the colours of the rainbow ”

“ Since black, when painted next to white, looks no blacker than when next to black; and white when next to black looks no whiter than white, as is seen by the images transmitted through a small hole or by the edges of any opaque screen ... ”

“ And if the end _n_ is red the eye _u_ on that side of the holes will not see the green colour of _o_, but only the red of _n_ according to the 7th of this where it is said: Every form projects images from itself by the shortest line, which necessarily is a straight line, &c.
[Footnote: 13. This probably refers to the diagram given under No. 66.] ”