INTRODUCTION TO PERSPECTIVE:--THAT IS OF THE FUNCTION OF THE EYE.

Behold here O reader! a thing concerning which we cannot trust our
forefathers, the ancients, who tried to define what the Soul and
Life are--which are beyond proof, whereas those things, which can at
any time be clearly known and proved by experience, remained for
many ages unknown or falsely understood. The eye, whose function we
so certainly know by experience, has, down to my own time, been
defined by an infinite number of authors as one thing; but I find,
by experience, that it is quite another. [Footnote 13: Compare the
note to No. 70.]

[Footnote: In section 13 we already find it indicated that the study
of Perspective and of Optics is to be based on that of the functions
of the eye. Leonardo also refers to the science of the eye, in his
astronomical researches, for instance in MS. F 25b '_Ordine del
provare la terra essere una stella: Imprima difinisce l'occhio'_,
&c. Compare also MS. E 15b and F 60b. The principles of astronomical
perspective.]