In Fig. 12., altering its proportions a little for the sake of
clearness, and putting it as here in Fig. 13., draw a horizontal line
_aR′_ and make _aR′_ equal to _aR_.
Through the points _R_ and _b_ draw _R′M_, cutting the sight-line in
_M_. Join _TV_. Now the reader will find experimentally that _VM_ is
equal to _VT_.[17]
[Illustration: Fig. 13.]
Hence it follows that, if from the vanishing-point _V_ we lay off on
the sight-line a distance, _VM_, equal to _VT_; then draw through _a_ a
horizontal line _aR′_, make _aR′_ equal to the sight-magnitude of _AB_,
and join _R′M_; the line _R′M_ will cut _aV_ in _b_. And this is in
practice generally the most convenient way of obtaining the length of
_ab_.