Full text: Gravesande, Willem Jacob: An essay on perspective

An ESSAY from the Eye O upon the Baſe Line, the Appear-
ances of all Lines paſſing through S, will be
perpendicular to the Baſe Line; but the ſaid
Point S is the Station Point. Whence, & c.

53. Problem II.

42. To throw a Line in the Geometrical Plane
into Perſpective.

It has been ſhewn , that to have the Repre- ſentation of a right Line, the Perſpective of
the Extremes of the ſaid Line, need only be
found; and although it is difficult to find the Repreſentation of two Points, nevertheleſs I
ſhall ſhew here how to find more eaſy the Re-
preſentation of a Line in ſome Caſes.

53.1.

21.
22.

1. Let A B be a Line parallel to the Baſe
Line: To draw the Repreſentation of which,
having firſt found the Point a the Appearance
of A, one of the Ends of the given Line; af-
terwards through that Appearance, draw a Pa-
rallel to the Baſe Line; then the Line B O,
drawn from B to the Eye O, will cut the ſaid
parallel in the Point b, and b a will be the Re-
preſentation ſought.

53.1.

Fig. 13.

43. 2. Let C G be a Line, which continued
out, cuts the baſe Line in E. Now to draw the
Appearance thereof; through the Eye O, draw a
Line parallel thereto, cutting the Horizontal
Line in D, and joyn the Points E and D, by
the Line E D, which cut in the Points c and g,
by Lines drawn from C and G to the Eye; then
the Part c g of the Line E D, is the Appearance
ſought.

54. Remark .

If the Lines G O and C O cut E D very ob-
liquely, and ſo their Interſection cannot be ex-

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