5023on PERSPECTIVE.
Perpendiculars f P, O H, muſt be let fall from
the Points f and O, on the Baſe Line, and the
Line P g drawn; then the Point V, wherein it
cuts the Perpendicular O H, is the Point of Sight
ſought, and the Parts O V, and V H determine
the Height and Diſtance of the Eye.
the Points f and O, on the Baſe Line, and the
Line P g drawn; then the Point V, wherein it
cuts the Perpendicular O H, is the Point of Sight
ſought, and the Parts O V, and V H determine
the Height and Diſtance of the Eye.
Method V.
35.
When the Appearance of a Point is known,
Let A be a Point in the Geometrical Plane,
11Fig. 11. and a its Repreſentation in the perſpective Plane,
it is requir’d to find the Appearance of the
Point B.
11Fig. 11. and a its Repreſentation in the perſpective Plane,
it is requir’d to find the Appearance of the
Point B.
Operation,
Without Compaſſes.
Without Compaſſes.
Draw a Line from the Point B to the Eye O,
and another from the Point E, wherein the
ſaid Line continued, cuts the Baſe Line, to the
Point A; then draw the Line E a, and where
it cuts B O, is the Point b ſought.
and another from the Point E, wherein the
ſaid Line continued, cuts the Baſe Line, to the
Point A; then draw the Line E a, and where
it cuts B O, is the Point b ſought.
Demonstration.
The Point E is its own Repreſentation;
and
becauſe the Point a is the Repreſentation of A,
the Line E a is that of E A. Now ſince the
Point B is in the Line E A, the Appearance of
this Point will be likewiſe in E a, as alſo 2227. B O; therefore it is in b the Interſection of the
Lines E a, and B O.
becauſe the Point a is the Repreſentation of A,
the Line E a is that of E A. Now ſince the
Point B is in the Line E A, the Appearance of
this Point will be likewiſe in E a, as alſo 2227. B O; therefore it is in b the Interſection of the
Lines E a, and B O.
Remark.
37.
If the Point A be in the Line B O, or
the Line B A be parallel, or a very little inclined
to the Baſe Line, we cannot then uſe this
the Line B A be parallel, or a very little inclined
to the Baſe Line, we cannot then uſe this