OF THE COLOURS OF SIMPLE DERIVED SHADOWS.

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'_,
while the red light causes the same body to cast a blue derived
shadow; but the primary shadow [on the dark side of the body itself]
is not of either of those hues, but a mixture of red and blue.

The derived shadows will be equal in depth if they are produced by
lights of equal strength and at an equal distance; this is proved.
[Footnote 53: The text is unfinished in the original.]

[Footnote: In the original diagram Leonardo has written within the
circle _q r corpo obroso_ (body in shadow); at the spot marked _A,
luminoso azzurro_ (blue luminous body); at _B, luminoso rosso_ (red
luminous body). At _E_ we read _ombra azzurra_ (blue tinted shadow)
and at _D ombra rossa_ (red tinted shadow).]

On the nature of colours (277. 278).