13. Keep in mind that the idea of a perfect shadow excludes all light, and that the space D, v, is an imperfect shadow, illumined, as we have seen, with a small portion of white light. Let this small portion of white light be considered as made up of red light and green light, according to what has been stated above, in Sect. 12, and the reason of the phenomenon will be readily understood. For we must now attend to the strong red light which passes through the glass c, and covers the paper everywhere, except in the space D, v, where it is intercepted: the effect of this strong light coming up to the very boundaries of the shadow D, v, is such as to incapacitate the eye from seeing at the same time the weaker red light contained in the shadow D, v, which we have proved to be really of a weak dull white colour, but which, because its red light cannot be seen, appears green to the eye.