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; as is shown by the lines d q o, and e r p. Hence

we may conclude that the shadow n m is exposed to the blue light

d e; but, as the red light a b cannot fall there, n m will

appear as a blue shadow on a red background tinted with blue,

because on the surface s c t d both lights can fall. But in the

shadows only one single light falls; for this reason these shadows

are of medium depth, since, if no light whatever mingled with the

shadow, it would be of the first degree of darkness &c. But in the

shadow at o p the blue light does not fall, because the body q r

interposes and intercepts it there. Only the red light a b falls

there and tinges the shadow of a red hue and so a ruddy shadow

appears on the background of mingled red and blue.

The shadow of q r at o p is red, being caused by the blue light

d e; and the shadow of q r at o' p' is blue being caused by

the red light a b. Hence we say that the blue light in this

instance causes a red derived shadow from the opaque body q' r',

205

The Notebooks of Leonardo Da Vinci Complete