To do this in perspective we pursue precisely the same method, as shown in this figure (143).
To measure a length Ag on the side of square AC, we draw a line from g to the point of sight S, and where it crosses diagonal AB at O we draw horizontal Og, and thus find the required depth Ag in the picture.
[ LXXVII]
How to Measure Distances by the Square and Diagonal
It may sometimes be convenient to have a ready method by which to measure the width and length of objects standing against the wall of a gallery, without referring to distance-points, &c.
Fig. 144.