Sacred Texts Age of Reason Index
III. Six Books on Light and Shade Index
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On the relative depth of cast shadows (200-202).A spot is most in the shade when a large number of darkened rays fall upon it. The spot which receives the rays at the widest angle and by darkened rays will be most in the dark; a will be twice as dark as b, because it originates from twice as large a base at an equal distance. A spot is most illuminated when a large number of luminous rays fall upon it. d is the beginning of the shadow d f, and tinges c but a little; d e is half of the shadow d f and gives a deeper tone where it is cast at b than at f. And the whole shaded space e gives its tone to the spot a. 99
111:99 : The diagram here referred to is on Pl. XLI, No. 2.