q.e.d.

Now let any right line meet four harmonicals in A, C, B, D, and if thoſe harmonicals be parallel, the thing is evident; but, if they interſect in V, draw ECF parallel to VD, the line moſt remote from C; then, by the firſt part, EC=CF, and AD:AC::VD:EC or CF. Therefore AD:AC::BD:CB. q.e.d.