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_. [Footnote:
The diagram here referred to is on Pl. XLI, No. 2.]