Let _n_ be the source of the shadow _e f_; it will assume its hue. Let _o_ be the source of _h e_ which will in the same way be tinged by its hue and so also the colour of _v h_ will be affected by _p_ which causes it; and the shadow of the triangle _z k y_ will be affected by the colour of _q_, because it is produced by it. [7] In proportion as _c d_ goes into _a d_, will _n r s_ be darker than _m_; and the rest of the space will be shadowless [11]. _f g_ is the highest light, because here the whole light of the window _a d_ falls; and thus on the opaque body _m e_ is in equally high light; _z k y_ is a triangle which includes the deepest shadow, because the light _a d_ cannot reach any part of it. _x h_ is the 2nd grade of shadow, because it receives only 1/3 of the light from the window, that is _c d_. The third grade of shadow is _h e_, where two thirds of the light from the window is visible. The last grade of shadow is _b d e f_, because the highest grade of light from the window falls at _f_. [Footnote: The diagram Pl. III, No. 1 belongs to this chapter as well as the text given in No. 148. Lines 7-11 (compare lines 8-12 of No. 148) which are written within the diagram, evidently apply to both sections and have therefore been inserted in both.] 276. 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.