INTRODUCTION.

[Having already treated of the nature of shadows and the way in
which they are cast [Footnote 2: _Avendo io tractato._--We may
suppose that he here refers to some particular MS., possibly Paris
C.], I will now consider the places on which they fall; and their
curvature, obliquity, flatness or, in short, any character I may be
able to detect in them.]

Shadow is the obstruction of light. Shadows appear to me to be of
supreme importance in perspective, because, without them opaque and
solid bodies will be ill defined; that which is contained within
their outlines and their boundaries themselves will be
ill-understood unless they are shown against a background of a
different tone from themselves. And therefore in my first
proposition concerning shadow I state that every opaque body is
surrounded and its whole surface enveloped in shadow and light. And
on this proposition I build up the first Book. Besides this, shadows
have in themselves various degrees of darkness, because they are
caused by the absence of a variable amount of the luminous rays; and
these I call Primary shadows because they are the first, and
inseparable from the object to which they belong. And on this I will
found my second Book. From these primary shadows there result
certain shaded rays which are diffused through the atmosphere and
these vary in character according to that of the primary shadows
whence they are derived. I shall therefore call these shadows
Derived shadows because they are produced by other shadows; and the
third Book will treat of these. Again these derived shadows, where
they are intercepted by various objects, produce effects as various
as the places where they are cast and of this I will treat in the
fourth Book. And since all round the derived shadows, where the
derived shadows are intercepted, there is always a space where the
light falls and by reflected dispersion is thrown back towards its
cause, it meets the original shadow and mingles with it and modifies
it somewhat in its nature; and on this I will compose my fifth Book.
Besides this, in the sixth Book I will investigate the many and
various diversities of reflections resulting from these rays which
will modify the original [shadow] by [imparting] some of the various
colours from the different objects whence these reflected rays are
derived. Again, the seventh Book will treat of the various distances
that may exist between the spot where the reflected rays fall and
that where they originate, and the various shades of colour which
they will acquire in falling on opaque bodies.

Different principles and plans of treatment (112--116).