[Footnote: 10. In the middle ages--for instance, by ROGER BACON, by
VITELLONE, with whose works Leonardo was certainly familiar, and by
all the writers of the Renaissance Perspective and Optics were not
regarded as distinct sciences. Perspective, indeed, is in its widest
application the science of seeing. Although to Leonardo the two
sciences were clearly separate, it is not so as to their names; thus
we find axioms in Optics under the heading Perspective. According to
this arrangement of the materials for the theoretical portion of the
_libro di pittura_ propositions in Perspective and in Optics stand
side by side or occur alternately. Although this particular chapter
deals only with Optics, it is not improbable that the words _partiro
la presente opera in 3 parti_ may refer to the same division into
three sections which is spoken of in chapters 14 to 17.].

The plan of the book on Painting (14--17).

14.

ON THE THREE BRANCHES OF PERSPECTIVE.

There are three branches of perspective; the first deals with the
reasons of the (apparent) diminution of objects as they recede from
the eye, and is known as Diminishing Perspective.--The second
contains the way in which colours vary as they recede from the eye.
The third and last is concerned with the explanation of how the
objects [in a picture] ought to be less finished in proportion as
they are remote (and the names are as follows):

Linear Perspective. The Perspective of Colour. The Perspective of
Disappearance.