.
So that, in order to determine the length of
a b
, we need not draw the long and distant line A B, but only
a
R parallel to it, and of its sight-magnitude; which is a great gain, for the line A B may be two miles long, and the line
a
R perhaps only two inches.
[p21]
][COROLLARY III.]
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