The construction above the eye is as in [Fig. 44.]; the roof is reversed in direction merely to familiarize the student with the different aspects of its lines.
[p53]
][PROBLEM XVI.]
TO FIND THE VANISHING-POINT OF A GIVEN INCLINED LINE.
If, in [Fig. 43.] or [Fig. 44.], the lines A Y and A′ Y′ be produced, the student will find that they meet.
Let P, [Fig. 45.], be the point at which they meet.
From P let fall the vertical P V on the sight-line, cutting the sight-line in V.
Then the student will find experimentally that V is the vanishing-point of the line A C.[Footnote 26] ]
Complete the rectangle of the base A C′, by drawing A′ C′ to V, and C C′ to the vanishing-point of A A′.
Join Y′ C′.
Now if Y C and Y′ C′ be produced downwards, the student will find that they meet.