y and the foot thereof.

At _S_ (point of sight) raise vertical _SF'_, making it equal to _fL_.
_F'_ becomes the foot of the luminary, whilst the luminary itself still
remains at _L_.

[Illustration: Fig. 273.]

We have but to turn this page half round and look at it from the right,
and we shall see that _SF'_ becomes as it were the horizontal line. The
luminary _L_ is at the right side of point _S_ instead of the left, and
the foot thereof is, as before, the trace of the luminary, as it is just
underneath it. We shall also see that by proceeding as in previous
figures we obtain the same results on the wall as we did on the
horizontal plane. Fig. B being on the horizontal plane is treated as
already shown. The steps have their shadows partly on the wall and
partly on the horizontal plane, so that the shadows on the wall are
outlined from _F'_ and those on the ground from _f_. Note shadow of roof
_A_, and how the line drawn from _F'_ through _A_ is met by the line
drawn from the luminary _L_, at the point _P_, and how the lower line of
the shadow is directed to point of sight _S_.

[Illustration: Fig. 274.]

Fig. 274 is a larger drawing of the steps, &c., in further illustration
of the above.

CLIII

SUN BEHIND THE PICTURE THROWING SHADOW ON AN INCLINED PLANE

[Illustration: Fig. 275.]

The vanishing point of the shadows on an inclined plane is on a vertical
dropped from the luminary to a point (_F_) on a level with the vanishing
point (_P_) of that inclined plane. Thus _P_ is the vanishing point of
the inclined plane _K_. Draw horizontal _PF_ to meet _fL_ (the line
drawn from the luminary to the horizon). Then _F_ will be the vanishing
point of the shadows on the inclined plane. To find the shadow of _M_
draw lines from _F_ through the base _eg_ to _cd_. From luminary _L_
draw lines through _ab_, also to _cd_, where they will meet those drawn
from _F_. Draw _CD_, which determines the length of the shadow _egcd_.

CLIV

THE SUN IN FRONT OF THE PICTURE

[Illustration: Fig. 276.]

When the sun is in front of the picture we have exactly the opposite
effect to that we have just been studying. The shadows, instead of
coming towards us, are retreating from us, and the objects throwing them
are in full light, consequently we have to reverse our treatment. Let us
suppose the sun to be placed above the horizon at _L'_, on the right of
the picture and behind the spectator (Fig. 276). If we