[Illustration: Fig. 12.]
Therefore, when the position of the point _A_ is fixed in _a_, as in
Fig. 12., and _aV_ is drawn to the vanishing-point; if we draw a line
_aR_ from _a_, parallel to _AB_, and make _aR_ equal to the
sight-magnitude of _AB_, and then join _RT_, the line _RT_ will cut _aV_
in _b_.
So that, in order to determine the length of _ab_, we need not draw the
long and distant line _AB_, but only _aR_ parallel to it, and of its
sight-magnitude; which is a great gain, for the line _AB_ may be two
miles long, and the line _aR_ perhaps only two inches.