Although the balls _a b c_ are lighted from one window,
nevertheless, if you follow the lines of their shadows you will see
they intersect at a point forming the angle _n_.
[Footnote: The diagram belonging to this passage is slightly
sketched on Pl. XXXII; a square with three balls below it. The first
three lines of the text belonging to it are written above the sketch
and the six others below it.]
138.
Every shadow cast by a body has a central line directed to a single
point produced by the intersection of luminous lines in the middle
of the opening and thickness of the window. The proposition stated
above, is plainly seen by experiment. Thus if you draw a place with
a window looking northwards, and let this be _s f_, you will see a
line starting from the horizon to the east, which, touching the 2
angles of the window _o f_, reaches _d_; and from the horizon on the
west another line, touching the other 2 angles _r s_, and ending at
_c_; and their intersection falls exactly in the middle of the
opening and thickness of the window. Again, you can still better
confirm this proof by placing two sticks, as shown at _g h_; and you
will see the line drawn from the centre of the shadow directed to
the centre _m_ and prolonged to the horizon _n f_.
[Footnote: _B_ here stands for _cerchio del' orizonte tramontano_ on
the original diagram (the circle of the horizon towards the North);
_A_ for _levante_ (East) and _C_ for _ponete_ (West).]
139.
Every shadow with all its variations, which becomes larger as its
distance from the object is greater, has its external lines
intersecting in the middle, between the light and the object. This
proposition is very evident and is confirmed by experience. For, if
_a b_ is a window without any object interposed, the luminous
atmosphere to the right hand at _a_ is seen to the left at _d_. And
the atmosphere at the left illuminates on the right at _c_, and the
lines intersect at the point _m_.
[Footnote: _A_ here stands for _levante_ (East), _B_ for _ponente_
(West).]
140.
Every body in light and shade is situated between 2 pyramids one
dark and the other luminous, one is visible the other is not. But
this only happens when the light enters by a window. Supposing _a b_
to be the window and _r_ the body in light and shade, the light to
the right hand _z_ will pass the object to the left and go on to
_p_; the light to the left at _k_ will pass to the right of the
object at _i_ and go on to _m_ and the two lines will intersect at
_c_ and form a pyramid. Then again _a_ _b_ falls on the shaded body
at _i_ _g_ and forms a pyramid _f_ _i_ _g_. _f_ will be dark because
the light _a_ _b_ can never fall there; _i_ _g_ _c_ will be
illuminated because the light falls upon it.
Light and shadow with regard to the position of the eye (141--145).
141.
Every shaded body that is larger than the pupil and that interposes
between the luminous body and the eye will be seen dark.
When the eye is placed between the luminous body and the objects
illuminated by it, these objects will be seen without any shadow.
[Footnote: The diagram which in the original stands above line 1 is
given on Plate II, No 2. Then, after a blank space of about eight
lines, the diagram Plate II No 3 is placed in the original. There is
no explanation of it beyond the one line written under it.]
142.
Why the 2 lights one on each side of a body having two pyramidal
sides of an obtuse apex leave it devoid of shadow.
[Footnote: The sketch illustrating this is on Plate XLI No 1.]
143.
A body in shadow situated between the light and the eye can never
display its illuminated portion unless the eye can see the whole of
the primary light.
[Footnote: _A_ stands for _corpo_ (body), _B_ for _lume_ (light).]
144.
The eye which looks (at a spot) half way between the shadow and the
light which surrounds the body in shadow will see that the deepest
shadows on that body will meet the eye at equal angles, that is at
the same angle as that of sight.
[Footnote: In both these diagrams _A_ stands for _lume_ (light) _B_
for _ombra_ (shadow).]
145.
OF THE DIFFERENT LIGHT AND SHADE IN VARIOUS ASPECTS AND OF OBJECTS
PLACED IN THEM.
If the sun is in the East and you look towards the West you will see
every thing in full light and totally without shadow because you see
them from the same side as the sun: and if you look towards the
South or North you will see all objects in light and shade, because
you see both the side towards the sun and the side away from it; and
if you look towards the coming of the sun all objects will show you
their shaded side, because on that side the sun cannot fall upon
them.
The law of the incidence of light.
146.
The edges of a window which are illuminated by 2 lights of equal
degrees of brightness will not reflect light of equal brightness
into the chamber within.
If _b_ is a candle and _a c_ our hemisphere both will illuminate the
edges of the window _m_ _n_, but light _b_ will only illuminate _f
g_ and the hemisphere _a_ will light all of _d e_.
147.