Of the Quality of Light.
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.