The Notebooks of Leonardo Da Vinci — Complete
c_. The ray of light will be weaker in proportion to its distance
from the hole through which it falls.
FIFTH BOOK ON LIGHT AND SHADE.
Principles of reflection (203. 204).
203.
OF THE WAY IN WHICH THE SHADOWS CAST BY OBJECTS OUGHT TO BE DEFINED.
If the object is the mountain here figured, and the light is at the
point _a_, I say that from _b d_ and also from _c f_ there will be
no light but from reflected rays. And this results from the fact
that rays of light can only act in straight lines; and the same is
the case with the secondary or reflected rays.
204.
The edges of the derived shadow are defined by the hues of the
illuminated objects surrounding the luminous body which produces the
shadow.
On reverberation.
205.
OF REVERBERATION.
Reverberation is caused by bodies of a bright nature with a flat and
semi opaque surface which, when the light strikes upon them, throw
it back again, like the rebound of a ball, to the former object.
WHERE THERE CAN BE NO REFLECTED LIGHTS.
All dense bodies have their surfaces occupied by various degrees of
light and shade. The lights are of two kinds, one called original,
the other borrowed. Original light is that which is inherent in the
flame of fire or the light of the sun or of the atmosphere. Borrowed
light will be reflected light; but to return to the promised
definition: I say that this luminous reverberation is not produced
by those portions of a body which are turned towards darkened
objects, such as shaded spots, fields with grass of various height,
woods whether green or bare; in which, though that side of each
branch which is turned towards the original light has a share of
that light, nevertheless the shadows cast by each branch separately
are so numerous, as well as those cast by one branch on the others,
that finally so much shadow is the result that the light counts for
nothing. Hence objects of this kind cannot throw any reflected light
on opposite objects.
Reflection on water (206. 207).
206.
PERSPECTIVE.
The shadow or object mirrored in water in motion, that is to say in
small wavelets, will always be larger than the external object
producing it.
207.
It is impossible that an object mirrored on water should correspond
in form to the object mirrored, since the centre of the eye is above
the surface of the water.
This is made plain in the figure here given, which demonstrates that
the eye sees the surface _a b_, and cannot see it at _l f_, and at
_r t_; it sees the surface of the image at _r t_, and does not see
it in the real object _c d_. Hence it is impossible to see it, as
has been said above unless the eye itself is situated on the surface
of the water as is shown below [13].
[Footnote: _A_ stands for _ochio_ [eye], _B_ for _aria_ [air], _C_
for _acqua_ [water], _D_ for _cateto_ [cathetus].--In the original
MS. the second diagram is placed below line 13.]
Experiments with the mirror (208-210).
208.
THE MIRROR.
If the illuminated object is of the same size as the luminous body
and as that in which the light is reflected, the amount of the
reflected light will bear the same proportion to the intermediate
light as this second light will bear to the first, if both bodies
are smooth and white.
209.
Describe how it is that no object has its limitation in the mirror
but in the eye which sees it in the mirror. For if you look at your
face in the mirror, the part resembles the whole in as much as the
part is everywhere in the mirror, and the whole is in every part of
the same mirror; and the same is true of the whole image of any
object placed opposite to this mirror, &c.
210.
No man can see the image of another man in a mirror in its proper
place with regard to the objects; because every object falls on [the
surface of] the mirror at equal angles. And if the one man, who sees
the other in the mirror, is not in a direct line with the image he
will not see it in the place where it really falls; and if he gets
into the line, he covers the other man and puts himself in the place
occupied by his image. Let _n o_ be the mirror, _b_ the eye of your
friend and _d_ your own eye. Your friend's eye will appear to you at
_a_, and to him it will seem that yours is at _c_, and the
intersection of the visual rays will occur at _m_, so that either of
you touching _m_ will touch the eye of the other man which shall be
open. And if you touch the eye of the other man in the mirror it
will seem to him that you are touching your own.
Appendix:--On shadows in movement (211. 212).
211.
OF THE SHADOW AND ITS MOTION.
When two bodies casting shadows, and one in front of the other, are
between a window and the wall with some space between them, the
shadow of the body which is nearest to the plane of the wall will
move if the body nearest to the window is put in transverse motion
across the window. To prove this let _a_ and _b_ be two bodies
placed between the window _n m_ and the plane surface _o p_ with
sufficient space between them as shown by the space _a b_. I say
that if the body _a_ is moved towards _s_ the shadow of the body _b_
which is at _c_ will move towards _d_.
212.
OF THE MOTION OF SHADOWS.
The motion of a shadow is always more rapid than that of the body
which produces it if the light is stationary. To prove this let _a_
be the luminous body, and _b_ the body casting the shadow, and _d_
the shadow. Then I say that in the time while the solid body moves
from _b_ to _c_, the shadow _d_ will move to _e_; and this
proportion in the rapidity of the movements made in the same space
of time, is equal to that in the length of the space moved over.
Thus, given the proportion of the space moved over by the body _b_
to _c_, to that moved over by the shadow _d_ to _e_, the proportion
in the rapidity of their movements will be the same.
But if the luminous body is also in movement with a velocity equal
to that of the solid body, then the shadow and the body that casts
it will move with equal speed. And if the luminous body moves more
rapidly than the solid body, the motion of the shadow will be slower
than that of the body casting it.
But if the luminous body moves more slowly than the solid body, then
the shadow will move more rapidly than that body.
SIXTH BOOK ON LIGHT AND SHADE.
The effect of rays passing through holes (213. 214).
213.
PERSPECTIVE.
If you transmit the rays of the sun through a hole in the shape of a
star you will see a beautiful effect of perspective in the spot
where the sun's rays fall.
[Footnote: In this and the following chapters of MS. C the order of
the original paging has been adhered to, and is shown in
parenthesis. Leonardo himself has but rarely worked out the subject
of these propositions. The space left for the purpose has
occasionally been made use of for quite different matter. Even the
numerous diagrams, most of them very delicately sketched, lettered
and numbered, which occur on these pages, are hardly ever explained,
with the exception of those few which are here given.]
214.
No small hole can so modify the convergence of rays of light as to
prevent, at a long distance, the transmission of the true form of
the luminous body causing them. It is impossible that rays of light
passing through a parallel [slit], should not display the form of
the body causing them, since all the effects produced by a luminous
body are [in fact] the reflection of that body: The moon, shaped
like a boat, if transmitted through a hole is figured in the surface
[it falls on] as a boatshaped object. [Footnote 8: In the MS. a
blank space is left after this question.] Why the eye sees bodies at
a distance, larger than they measure on the vertical plane?.
[Footnote: This chapter, taken from another MS. may, as an
exception, be placed here, as it refers to the same subject as the
preceding section.]
On gradation of shadows (215. 216).
215.
Although the breadth and length of lights and shadow will be
narrower and shorter in foreshortening, the quality and quantity of
the light and shade is not increased nor diminished.
[3]The function of shade and light when diminished by
foreshortening, will be to give shadow and to illuminate an object
opposite, according to the quality and quantity in which they fall
on the body.
[5]In proportion as a derived shadow is nearer to its penultimate
extremities the deeper it will appear, _g z_ beyond the intersection
faces only the part of the shadow [marked] _y z_; this by
intersection takes the shadow from _m n_ but by direct line it takes
the shadow _a m_ hence it is twice as deep as _g z_. _Y x_, by
intersection takes the shadow _n o_, but by direct line the shadow
_n m a_, therefore _x y_ is three times as dark as _z g_; _x f_, by
intersection faces _o b_ and by direct line _o n m a_, therefore we
must say that the shadow between _f x_ will be four times as dark as
the shadow _z g_, because it faces four times as much shadow.
Let _a b_ be the side where the primary shadow is, and _b c_ the
primary light, _d_ will be the spot where it is intercepted,_f g_
the derived shadow and _f e_ the derived light.
And this must be at the beginning of the explanation.
[Footnote: In the original MS. the text of No. 252 precedes the one
given here. In the text of No. 215 there is a blank space of about
four lines between the lines 2 and 3. The diagram given on Pl. VI,
No. 2 is placed between lines 4 and 5. Between lines 5 and 6 there
is another space of about three lines and one line left blank
between lines 8 and 9. The reader will find the meaning of the whole
passage much clearer if he first reads the final lines 11--13.
Compare also line 4 of No. 270.]
On relative proportion of light and shadows (216--221).
216.
That part of the surface of a body on which the images [reflection]
from other bodies placed opposite fall at the largest angle will
assume their hue most strongly. In the diagram below, 8 is a larger
angle than 4, since its base _a n_ is larger than _e n_ the base of
4. This diagram below should end at _a n_ 4 8. [4]That portion of
the illuminated surface on which a shadow is cast will be brightest
which lies contiguous to the cast shadow. Just as an object which is
lighted up by a greater quantity of luminous rays becomes brighter,
so one on which a greater quantity of shadow falls, will be darker.
Let 4 be the side of an illuminated surface 4 8, surrounding the
cast shadow _g e_ 4. And this spot 4 will be lighter than 8, because
less shadow falls on it than on 8. Since 4 faces only the shadow _i
n_; and 8 faces and receives the shadow _a e_ as well as _i n_ which
makes it twice as dark. And the same thing happens when you put the
atmosphere and the sun in the place of shade and light.
[12] The distribution of shadow, originating in, and limited by,
plane surfaces placed near to each other, equal in tone and directly
opposite, will be darker at the ends than at the beginning, which
will be determined by the incidence of the luminous rays. You will
find the same proportion in the depth of the derived shadows _a n_
as in the nearness of the luminous bodies _m b_, which cause them;
and if the luminous bodies were of equal size you would still
farther find the same proportion in the light cast by the luminous
circles and their shadows as in the distance of the said luminous
bodies.
[Footnote: The diagram originally placed between lines 3 and 4 is on
Pl. VI, No. 3. In the diagram given above line 14 of the original,
and here printed in the text, the words _corpo luminoso_ [luminous
body] are written in the circle _m_, _luminoso_ in the circle _b_
and _ombroso_ [body in shadow] in the circle _o_.]
217.
THAT PART OF THE REFLECTION WILL BE BRIGHTEST WHERE THE REFLECTED