on PERSPECTIVE.
fore the Point X is the Repreſentation of the

ſaid Extremity.

##
147.
Corollary
.

94. It is manifeſt from hence, that when the

Perſpective of a Line, perpendicular to the

Geometrical Plane is once found, it is eaſy af-

terwards to find the Repreſentations of any Per-

pendiculars of the ſame Length as that.

##
148.
Method
II.

The Perſpective Plane being conſider’d as the Geo-

metrical Plane.

95. Let T (as in Fig. 51.) be the accidental

Point of perpendicular Lines to the Geometri-

cal Plane; H I the Arc of a Circle, whoſe Cen-

ter is T, and ſemidiameter the Eye’s Diſtance

from the perſpective Plane: Alſo let a be the

Point where the Perpendicular, whoſe Appear-

ance is ſought, meets the perſpective Plane, and

B C the Length of this Perpendicular.

##
149.
Operation
,

About the Point a, as a Center, and with the

Semidiameter B C, deſcribe the Circle L F, and

draw the Line I L, or H F, touching each of the

Circles H I, and F L; and then a X or a x, is

the Appearance ſought, viz. a X, when the Per-

pendicular is above that Surface of the per-

ſpective Plane next to the Eye, and a x, when

the Perpendicular is on the oppoſite Side.

##
150.
Demonstration
.

Draw the Radii a F, a L, T I, T H, to the

Points of Contact F, L, I, and H. Then be-