, 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