Notebooks of Leonardo da Vinci

gateway; and without this nothing can be done well in the matter of
drawing.

20.

The painter who draws merely by practice and by eye, without any
reason, is like a mirror which copies every thing placed in front of
it without being conscious of their existence.

The function of the eye (21-23).

21.

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.]

22.

Here [in the eye] forms, here colours, here the character of every
part of the universe are concentrated to a point; and that point is
so marvellous a thing ... Oh! marvellous, O stupendous Necessity--by
thy laws thou dost compel every effect to be the direct result of
its cause, by the shortest path. These [indeed] are miracles;...

In so small a space it can be reproduced and rearranged in its whole
expanse. Describe in your anatomy what proportion there is between
the diameters of all the images in the eye and the distance from
them of the crystalline lens.

23.

OF THE 10 ATTRIBUTES OF THE EYE, ALL CONCERNED IN PAINTING.

Painting is concerned with all the 10 attributes of sight; which
are:--Darkness, Light, Solidity and Colour, Form and Position,
Distance and Propinquity, Motion and Rest. This little work of mine
will be a tissue [of the studies] of these attributes, reminding the
painter of the rules and methods by which he should use his art to
imitate all the works of Nature which adorn the world.

24.

ON PAINTING.

Variability of the eye.

1st. The pupil of the eye contracts, in proportion to the increase
of light which is reflected in it. 2nd. The pupil of the eye expands
in proportion to the diminution in the day light, or any other
light, that is reflected in it. 3rd. [Footnote: 8. The subject of
this third proposition we find fully discussed in MS. G. 44a.]. The
eye perceives and recognises the objects of its vision with greater
intensity in proportion as the pupil is more widely dilated; and
this can be proved by the case of nocturnal animals, such as cats,
and certain birds--as the owl and others--in which the pupil varies
in a high degree from large to small, &c., when in the dark or in
the light. 4th. The eye [out of doors] in an illuminated atmosphere
sees darkness behind the windows of houses which [nevertheless] are
light. 5th. All colours when placed in the shade appear of an equal
degree of darkness, among themselves. 6th. But all colours when
placed in a full light, never vary from their true and essential
hue.

25.

OF THE EYE.

Focus of sight.

If the eye is required to look at an object placed too near to it,
it cannot judge of it well--as happens to a man who tries to see the
tip of his nose. Hence, as a general rule, Nature teaches us that an
object can never be seen perfectly unless the space between it and
the eye is equal, at least, to the length of the face.

Differences of perception by one eye and by both eyes (26-29).

26.

OF THE EYE.

When both eyes direct the pyramid of sight to an object, that object
becomes clearly seen and comprehended by the eyes.