shade modify every portion of the light, except where it is
strongest, at _c_. To prove this let _d a_ be the primary shadow
which is turned towards the point _e_, and darkens it by its derived
shadow; as may be seen by the triangle _a e d_, in which the
angle _e_ faces the darkened base _d a e_; the point _v_ faces the
dark shadow _a s_ which is part of _a d_, and as the whole is
greater than a part, _e_ which faces the whole base [of the
triangle], will be in deeper shadow than _v_ which only faces part
of it. In consequence of the conclusion [shown] in the above
diagram, _t_ will be less darkened than _v_, because the base of the
_t_ is part of the base of the _v_; and in the same way it follows
that _p_ is less in shadow than _t_, because the base of the _p_ is
part of the base of the _t_. And _c_ is the terminal point of the
derived shadow and the chief beginning of the highest light.

[Footnote: The diagram on Pl. IV, No. 5 belongs to this passage; but
it must be noted that the text explains only the figure on the
right-hand side.]

FOURTH BOOK ON LIGHT AND SHADE.

On the shape of the cast shadows (188-191).

188.

The form of the shadow cast by any body of uniform density can never
be the same as that of the body producing it. [Footnote: Comp. the
drawing on PI. XXVIII, No. 5.]

189.

No cast shadow can produce the true image of the body which casts it
on a vertical plane unless the centre of the light is equally
distant from all the edges of that body.

190.

If a window _a b_ admits the sunlight into a room, the sunlight will
magnify the size of the window and diminish the shadow of a man in
such a way as that when the man makes that dim shadow of himself,
approach to that which defines the real size of the window, he will
see the shadows where they come into contact, dim and confused from
the strength of the light, shutting off and not allowing the solar
rays to pass; the effect of the shadow of the man cast by this
contact will be exactly that figured above.

[Footnote: It is scarcely possible to render the meaning of this
sentence with strict accuracy; mainly because the grammatical
construction is defective in the most important part--line 4. In the
very slight original sketch the shadow touches the upper arch of the
window and the correction, here given is perhaps not justified.]

191.

A shadow is never seen as of uniform depth on the surface which
intercepts it unless every portion of that surface is equidistant
from the luminous body. This is proved by the 7th which says:--The
shadow will appear lighter or stronger as it is surrounded by a
darker or a lighter background. And by the 8th of this:--The
background will be in parts darker or lighter, in proportion as it
is farther from or nearer to the luminous body. And:--Of various
spots equally distant from the luminous body those will always be in
the highest light on which the rays fall at the smallest angles: The
outline of the shadow as it falls on inequalities in the surface
will be seen with all the contours similar to those of the body that
casts it, if the eye is placed just where the centre of the light
was.

The shadow will look darkest where it is farthest from the body that
casts it. The shadow _c d_, cast by the body in shadow _a b_ which
is equally distant in all parts, is not of equal depth because it is
seen on a back ground of varying brightness. [Footnote: Compare the
three diagrams on Pl. VI, no 1 which, in the original accompany this
section.]

On the outlines of cast shadows (192-195).

192.

The edges of a derived shadow will be most distinct where it is cast
nearest to the primary shadow.

193.

As the derived shadow gets more distant from the primary shadow, the
more the cast shadow differs from the primary shadow.

194.

OF SHADOWS WHICH NEVER COME TO AN END.

The greater the difference between a light and the body lighted by
it, the light being the larger, the more vague will be the outlines
of the shadow of that object.

The derived shadow will be most confused towards the edges of its
interception by a plane, where it is remotest from the body casting
it.

195.


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