b
draw
m
Q, cutting P Q in Q.
Then, by the proof already given in [page 20], P Q = P T.
Therefore if P is the vanishing-point of an inclined line A B, and Q P is a horizontal line drawn through it, make P Q equal to P T, and
a m
on the measuring-line equal to the sight-magnitude of the line A B in the diagram, and the line joining
m
Q will cut