The Notebooks of Leonardo Da Vinci (8 page)

79.

Only one line of the image, of all those that reach the visual
virtue, has no intersection; and this has no sensible dimensions
because it is a mathematical line which originates from a
mathematical point, which has no dimensions.

According to my adversary, necessity requires that the central line
of every image that enters by small and narrow openings into a dark
chamber shall be turned upside down, together with the images of the
bodies that surround it.

80.

It is impossible that the line should intersect itself; that is,
that its right should cross over to its left side, and so, its left
side become its right side. Because such an intersection demands two
lines, one from each side; for there can be no motion from right to
left or from left to right in itself without such extension and
thickness as admit of such motion. And if there is extension it is
no longer a line but a surface, and we are investigating the
properties of a line, and not of a surface. And as the line, having
no centre of thickness cannot be divided, we must conclude that the
line can have no sides to intersect each other. This is proved by
the movement of the line
a f
to
a b
and of the line
e b
to
e
f
, which are the sides of the surface
a f e b
. But if you move
the line
a b
and the line
e f
, with the frontends
a e
, to the
spot
c
, you will have moved the opposite ends
f b
towards each
other at the point
d
. And from the two lines you will have drawn
the straight line
c d
which cuts the middle of the intersection of
these two lines at the point
n
without any intersection. For, you
imagine these two lines as having breadth, it is evident that by
this motion the first will entirely cover the other—being equal
with it—without any intersection, in the position
c d
. And this
is sufficient to prove our proposition.

81.

Just as all lines can meet at a point without interfering with each
other—being without breadth or thickness—in the same way all the
images of surfaces can meet there; and as each given point faces the
object opposite to it and each object faces an opposite point, the
converging rays of the image can pass through the point and diverge
again beyond it to reproduce and re-magnify the real size of that
image. But their impressions will appear reversed—as is shown in
the first, above; where it is said that every image intersects as it
enters the narrow openings made in a very thin substance.

Read the marginal text on the other side.

In proportion as the opening is smaller than the shaded body, so
much less will the images transmitted through this opening intersect
each other. The sides of images which pass through openings into a
dark room intersect at a point which is nearer to the opening in
proportion as the opening is narrower. To prove this let
a b
be an
object in light and shade which sends not its shadow but the image
of its darkened form through the opening
d e
which is as wide as
this shaded body; and its sides
a b
, being straight lines (as has
been proved) must intersect between the shaded object and the
opening; but nearer to the opening in proportion as it is smaller
than the object in shade. As is shown, on your right hand and your
left hand, in the two diagrams
a
b
c
n
m
o
where, the
right opening
d
e
, being equal in width to the shaded object
a
b
, the intersection of the sides of the said shaded object occurs
half way between the opening and the shaded object at the point
c
.
But this cannot happen in the left hand figure, the opening
o
being much smaller than the shaded object
n
m
.

It is impossible that the images of objects should be seen between
the objects and the openings through which the images of these
bodies are admitted; and this is plain, because where the atmosphere
is illuminated these images are not formed visibly.

When the images are made double by mutually crossing each other they
are invariably doubly as dark in tone. To prove this let
d
e
h
be such a doubling which although it is only seen within the space
between the bodies in
b
and
i
this will not hinder its being
seen from
f
g
or from
f
m
; being composed of the images
a
b
i
k
which run together in
d
e
h
.

[Footnote: 81. On the original diagram at the beginning of this
chapter Leonardo has written "
azurro
" (blue) where in the
facsimile I have marked
A
, and "
giallo
" (yellow) where
B
stands.]

[Footnote: 15—23. These lines stand between the diagrams I and III.]

[Footnote: 24—53. These lines stand between the diagrams I and II.]

[Footnote: 54—97 are written along the left side of diagram I.]

82.

An experiment showing that though the pupil may not be moved from
its position the objects seen by it may appear to move from their
places.

If you look at an object at some distance from you and which is
below the eye, and fix both your eyes upon it and with one hand
firmly hold the upper lid open while with the other you push up the
under lid—still keeping your eyes fixed on the object gazed at—you
will see that object double; one [image] remaining steady, and the
other moving in a contrary direction to the pressure of your finger
on the lower eyelid. How false the opinion is of those who say that
this happens because the pupil of the eye is displaced from its
position.

How the above mentioned facts prove that the pupil acts upside down
in seeing.

[Footnote: 82. 14—17. The subject indicated by these two headings is
fully discussed in the two chapters that follow them in the
original; but it did not seem to me appropriate to include them
here.]

Demostration of perspective by means of a vertical glass plane
(83-85).

83.

Perspective is nothing else than seeing place [or objects] behind a
plane of glass, quite transparent, on the surface of which the
objects behind that glass are to be drawn. These can be traced in
pyramids to the point in the eye, and these pyramids are intersected
on the glass plane.

84.

Pictorial perspective can never make an object at the same distance,
look of the same size as it appears to the eye. You see that the
apex of the pyramid
f c d
is as far from the object
c
d
as the
same point
f
is from the object
a
b
; and yet
c
d
, which is
the base made by the painter's point, is smaller than
a
b
which
is the base of the lines from the objects converging in the eye and
refracted at
s
t
, the surface of the eye. This may be proved by
experiment, by the lines of vision and then by the lines of the
painter's plumbline by cutting the real lines of vision on one and
the same plane and measuring on it one and the same object.

85.

The vertical plane is a perpendicular line, imagined as in front of
the central point where the apex of the pyramids converge. And this
plane bears the same relation to this point as a plane of glass
would, through which you might see the various objects and draw them
on it. And the objects thus drawn would be smaller than the
originals, in proportion as the distance between the glass and the
eye was smaller than that between the glass and the objects.

The different converging pyramids produced by the objects, will
show, on the plane, the various sizes and remoteness of the objects
causing them.

All those horizontal planes of which the extremes are met by
perpendicular lines forming right angles, if they are of equal width
the more they rise to the level of eye the less this is seen, and
the more the eye is above them the more will their real width be
seen.

The farther a spherical body is from the eye the more you will see
of it.

The angle of sight varies with the distance (86-88)

86.

A simple and natural method; showing how objects appear to the eye
without any other medium.

The object that is nearest to the eye always seems larger than
another of the same size at greater distance. The eye
m
, seeing
the spaces
o v x
, hardly detects the difference between them, and
the. reason of this is that it is close to them [Footnote 6: It is
quite inconceivable to me why M. RAVAISSON, in a note to his French
translation of this simple passage should have remarked:
Il est
clair que c'est par erreur que Leonard a �crit
per esser visino
au
lieu de
per non esser visino. (See his printed ed. of MS. A. p.
38.)]; but if these spaces are marked on the vertical plane
n o
the space
o v
will be seen at
o r
, and in the same way the space
v x
will appear at
r q
. And if you carry this out in any place
where you can walk round, it will look out of proportion by reason
of the great difference in the spaces
o r
and
r q
. And this
proceeds from the eye being so much below [near] the plane that the
plane is foreshortened. Hence, if you wanted to carry it out, you
would have [to arrange] to see the perspective through a single hole
which must be at the point
m
, or else you must go to a distance of
at least 3 times the height of the object you see. The plane
o p
being always equally remote from the eye will reproduce the objects
in a satisfactory way, so that they may be seen from place to place.

87.

How every large mass sends forth its images, which may diminish
through infinity.

The images of any large mass being infinitely divisible may be
infinitely diminished.

88.

Objects of equal size, situated in various places, will be seen by
different pyramids which will each be smaller in proportion as the
object is farther off.

89.

Perspective, in dealing with distances, makes use of two opposite
pyramids, one of which has its apex in the eye and the base as
distant as the horizon. The other has the base towards the eye and
the apex on the horizon. Now, the first includes the [visible]
universe, embracing all the mass of the objects that lie in front of
the eye; as it might be a vast landscape seen through a very small
opening; for the more remote the objects are from the eye, the
greater number can be seen through the opening, and thus the pyramid
is constructed with the base on the horizon and the apex in the eye,
as has been said. The second pyramid is extended to a spot which is
smaller in proportion as it is farther from the eye; and this second
perspective [= pyramid] results from the first.

90.

Simple perspective is that which is constructed by art on a vertical
plane which is equally distant from the eye in every part. Complex
perspective is that which is constructed on a ground-plan in which
none of the parts are equally distant from the eye.

91.

No surface can be seen exactly as it is, if the eye that sees it is
not equally remote from all its edges.

92.

When an object opposite the eye is brought too close to it, its
edges must become too confused to be distinguished; as it happens
with objects close to a light, which cast a large and indistinct
shadow, so is it with an eye which estimates objects opposite to it;
in all cases of linear perspective, the eye acts in the same way as
the light. And the reason is that the eye has one leading line (of
vision) which dilates with distance and embraces with true
discernment large objects at a distance as well as small ones that
are close. But since the eye sends out a multitude of lines which
surround this chief central one and since these which are farthest
from the centre in this cone of lines are less able to discern with
accuracy, it follows that an object brought close to the eye is not
at a due distance, but is too near for the central line to be able
to discern the outlines of the object. So the edges fall within the
lines of weaker discerning power, and these are to the function of
the eye like dogs in the chase which can put up the game but cannot
take it. Thus these cannot take in the objects, but induce the
central line of sight to turn upon them, when they have put them up.
Hence the objects which are seen with these lines of sight have
confused outlines.

The relative size of objects with regard to their distance from the
eye (93-98).

93.

Small objects close at hand and large ones at a distance, being seen
within equal angles, will appear of the same size.

94.

There is no object so large but that at a great distance from the
eye it does not appear smaller than a smaller object near.

95.

Among objects of equal size that which is most remote from the eye
will look the smallest. [Footnote: This axiom, sufficiently clear in
itself, is in the original illustrated by a very large diagram,
constructed like that here reproduced under No. 108.