of
sciagraphy. A young shepherd was in love with the daughter of a potter,
but it so happened that they had to part, and were passing their last
evening together, when the girl, seeing the shadow of her lover's
profile cast from a lamp on to some wet plaster or on the wall, took a
metal point, perhaps some sort of iron needle, and traced the outline of
the face she loved on to the plaster, following carefully the outline of
the features, being naturally anxious to make it as like as possible.
The old potter, the father of the girl, was so struck with it that he
began to ornament his wares by similar devices, which gave them
increased value by the novelty and beauty thus imparted to them.

Here then we have a very good illustration of our present subject and
its three elements. First, the light shining on the wall; second, the
wall or the plane of projection, or plane of shade; and third, the
intervening object, which receives as much light on itself as it
deprives the wall of. So that the dark portion thus caused on the plane
of shade is the cast shadow of the intervening object.

We have to consider two sorts of shadows: those cast by a luminary a
long way off, such as the sun; and those cast by artificial light, such
as a lamp or candle, which is more or less close to the object. In the
first case there is no perceptible divergence of rays, and the outlines
of the sides of the shadows of regular objects, as cubes, posts, &c.,
will be parallel. In the second case, the rays diverge according to the
nearness of the light, and consequently the lines of the shadows,
instead of being parallel, are spread out.

CXLVIII

THE TWO KINDS OF SHADOWS

In Figs. 261 and 262 is seen the shadow cast by the sun by parallel
rays.

Fig. 263 shows the shadows cast by a candle or lamp, where the rays
diverge from the point of light to meet corresponding diverging lines
which start from the foot of the luminary on the ground.

[Illustration: Fig. 261.]

[Illustration: Fig. 262.]

The simple principle of cast shadows is that the rays coming from the
point of light or luminary pass over the top of the intervening object
which casts the shadow on to the plane of shade to meet the horizontal
trace of those rays on that plane, or the lines of light proceed from
the point of light, and the lines of the shadow are drawn from the foot
or trace of the point of light.

[Illustration: Fig. 263.]

[Illustration: Fig. 264.