Noble, Thomas; Clark, John Heaviside [Ill.]: Practical Perspective, Exemplified On Landscapes (London, 1809)

PRACTICAL PERSPECTIVE.

15

IX. The point X of the nearest original pyramid has the ray X Z D, proceeding
from it to the eye of the spectator, and it touches the plane or surface of the picture in
the point Z.

Let X Y be a line perpendicular to the plane of the picture: it will then be the
distance of the point X from the plane of the picture : it will also be parallel to C D, which
represents the dista?\ce of the spectator from the plane of the picture. Its perspective re-
presentation will (by obscr ill. of this section) tend to the point of sight C : therefore, sup-
pose the line Y C to be drawn on the plane of the picture: then you may observe that
the ray X D performs on the line Y C the operation taught by the IXth Problem of the
preceding Geometry ; it divides the line Y C into two parts, proportional to the two
lines X Y and D C : thus the distance of the point X (that is X Y) is to Y Z, as the distance
of the eye at D (that is D C) is to C Z, the remainder of the line Y C.

Y Z is therefore the perspective representation of the line X Y, which is the distance
of the point X, and Z is the perspective point for the original point X : Z is also the
point which the ray X Z D (see ohser. VIII.) must necessarily make upon the surface of
the picture..

The learner is desired to pay particular attention to this last observation, as it contains
all the demonstration necessary to enable him to understand the following plates,
and will therefore be frequently referred to, that he may not be fatigued by useless
repetition.

SECOND