Derived shadows are of three kinds of which one is spreading, the
second columnar, the third converging to the point where the two
sides meet and intersect, and beyond this intersection the sides are
infinitely prolonged or straight lines. And if you say, this shadow
must terminate at the angle where the sides meet and extend no
farther, I deny this, because above in the first on shadow I have
proved: that a thing is completely terminated when no portion of it
goes beyond its terminating lines. Now here, in this shadow, we see
the converse of this, in as much as where this derived shadow
originates we obviously have the figures of two pyramids of shadow
which meet at their angles. Hence, if, as [my] opponent says, the
first pyramid of shadow terminates the derivative shadow at the
angle whence it starts, then the second pyramid of shadow--so says
the adversary--must be caused by the angle and not from the body in
shadow; and this is disproved with the help of the 2nd of this which
says: Shadow is a condition produced by a body casting a shadow, and
interposed between this shadow and the luminous body. By this it is
made clear that the shadow is not produced by the angle of the
derived shadow but only by the body casting the shadow; &c. If a
spherical solid body is illuminated by a light of elongated form the
shadow produced by the longest portion of this light will have less
defined outlines than that which is produced by the breadth of the
same light. And this is proved by what was said before, which is:
That a shadow will have less defined outlines in proportion as the
light which causes it is larger, and conversely, the outlines are
clearer in proportion as it is smaller.
[Footnote: The two diagrams to this chapter are on Plate IV, No. 1.]
On the relation of derived and primary shadow (163-165).
163.
The derived shadow can never resemble the body from which it
proceeds unless the light is of the same form and size as the body
causing the shadow.
The derived shadow cannot be of the same form as the primary shadow
unless it is intercepted by a plane parallel to it.
164.
HOW A CAST SHADOW CAN NEVER BE OF THE SAME SIZE AS THE BODY THAT
CASTS IT.
If the rays of light proceed, as experience shows, from a single
point and are diffused in a sphere round this point, radiating and
dispersed through the air, the farther they spread the wider they
must spread; and an object placed between the light and a wall is
always imaged larger in its shadow, because the rays that strike it
[Footnote: 7. The following lines are wanting to complete the
logical connection.] would, by the time they have reached the wall,
have become larger.
165.
Any shadow cast by a body in light and shade is of the same nature
and character as that which is inseparable from the body. The centre
of the length of a shadow always corresponds to that of the luminous
body [Footnote 6: This second statement of the same idea as in the
former sentence, but in different words, does not, in the original,
come next to the foregoing; sections 172 and 127 are placed between
them.]. It is inevitable that every shadow must have its centre in a
line with the centre of the light.
On the shape of derived shadows (166-174).
166.