Gradations of strength in the shadows (148. 149).
148.
THAT PORTION OF A BODY IN LIGHT AND SHADE WILL BE LEAST LUMINOUS
WHICH IS SEEN UNDER THE LEAST AMOUNT OF LIGHT.
That part of the object which is marked _m_ is in the highest light
because it faces the window _a d_ by the line _a f_; _n_ is in the
second grade because the light _b d_ strikes it by the line _b e_;
_o_ is in the third grade, as the light falls on it from _c d_ by
the line _c h_; _p_ is the lowest light but one as _c d_ falls on it
by the line _d v_; _q_ is the deepest shadow for no light falls on
it from any part of the window.
In proportion as _c d_ goes into _a d_ so will _n r s_ be darker
than _m_, and all the rest is space without shadow.
[Footnote: The diagram belonging to this chapter is No. 1 on Plate
III. The letters _a b e d_ and _r_ are not reproduced in facsimile
of the original, but have been replaced by ordinary type in the
margin. 5-12. The original text of these lines is reproduced within
the diagram.--Compare No 275.]
149.
The light which falls on a shaded body at the acutest angle receives
the highest light, and the darkest portion is that which receives it
at an obtuse angle and both the light and the shadow form pyramids.
The angle _c_ receives the highest grade of light because it is
directly in front of the window _a b_ and the whole horizon of the
sky _m x_. The angle _a_ differs but little from _c_ because the
angles which divide it are not so unequal as those below, and only
that portion of the horizon is intercepted which lies between _y_
and _x_. Although it gains as much on the other side its line is
nevertheless not very strong because one angle is smaller than its
fellow. The angles _e i_ will have less light because they do not
see much of the light _m s_ and the light _v x_ and their angles are
very unequal. Yhe angle _k_ and the angle _f_ are each placed
between very unequal angles and therefore have but little light,
because at _k_ it has only the light _p t_, and at _f_ only _t q_;
_o g_ is the lowest grade of light because this part has no light at
all from the sky; and thence come the lines which will reconstruct a
pyramid that is the counterpart of the pyramid _c_; and this pyramid
_l_ is in the first grade of shadow; for this too is placed between
equal angles directly opposite to each other on either side of a
straight line which passes through the centre of the body and goes
to the centre of the light. The several luminous images cast within
the frame of the window at the points _a_ and _b_ make a light which
surrounds the derived shadow cast by the solid body at the points 4
and 6. The shaded images increase from _o g_ and end at 7 and 8.
[Footnote: The diagram belonging to this chapter is No. 2 on Plate
III. In the original it is placed between lines 3 and 4, and in the
reproduction these are shown in part. The semi circle above is
marked _orizonte_ (horizon). The number 6 at the left hand side,
outside the facsimile, is in the place of a figure which has become
indistinct in the original.]
On the intensity of shadows as dependent on the distance from the
light (150-152).
150.
The smaller the light that falls upon an object the more shadow it
will display. And the light will illuminate a smaller portion of the
object in proportion as it is nearer to it; and conversely, a larger
extent of it in proportion as it is farther off.
A light which is smaller than the object on which it falls will
light up a smaller extent of it in proportion as it is nearer to it,
and the converse, as it is farther from it. But when the light is
larger than the object illuminated it will light a larger extent of
the object in proportion as it is nearer and the converse when they
are farther apart.
151.
That portion of an illuminated object which is nearest to the source
of light will be the most strongly illuminated.
152.
That portion of the primary shadow will be least dark which is
farthest from the edges.
The derived shadow will be darker than the primary shadow where it
is contiguous with it.
On the proportion of light and shade (153-157).
153.
That portion of an opaque body will be more in shade or more in
light, which is nearer to the dark body, by which it is shaded, or
to the light that illuminates it.
Objects seen in light and shade show in greater relief than those
which are wholly in light or in shadow.
154.