The Elements of Perspective / arranged for the use of schools and intended to be read in connection with the first three books of Euclid

by John Ruskin

a

P in

b

.

We have now, therefore, to consider what relation the length of the line A B in this diagram, [Fig. 77.], has to the length of the line A B in reality.

Now the line A E in [Fig. 77.] represents the length of A E in reality.

But the angle A E B, [Fig. 77.], and the corresponding angle in all the constructions of the earlier problems, is in reality a right angle, though in the diagram necessarily represented as obtuse.

Fig. 79.

Therefore, if from E we draw E C, as in [Fig. 79.], at right angles to A E, make E C = E B, and join A C, A C will be the real length of the line A B.