In proportion to the number of times that _a b_ goes into _c d_ will
it be more luminous than _c d_. And similarly, in proportion as the
point _e_ goes into _c d_ will it be more luminous than _c d;_ and
this light is useful for carvers of delicate work. [Footnote 5: For
the same reason a window thus constructed would be convenient for an
illuminator or a miniature painter.]
[Footnote: M. RAVAISSON in his edition of the Paris MS. A remarks on
this passage: _"La figure porte les lettres_ f _et_ g, _auxquelles
rien ne renvoie dans l'explication; par consequent, cette
explication est incomplete. La figure semblerait, d'ailleurs, se
rapporter a l'effet de la reflexion par un miroir concave."_ So far
as I can see the text is not imperfect, nor is the sense obscure. It
is hardly necessary to observe that _c d_ here indicate the wall of
the room opposite to the window _e_ and the semicircle described by
_f g_ stands for the arch of the sky; this occurs in various
diagrams, for example under 511. A similar semicircle, Pl III, No. 2
(and compare No. 149) is expressly called '_orizonte_' in writing.]
519.
That the light should fall upon a picture from one window only. This
may be seen in the case of objects in this form. If you want to
represent a round ball at a certain height you must make it oval in
this shape, and stand so far off as that by foreshortening it
appears round.
520.