Source : Webster's Revised Unabridged Dictionary (1913)

Diagonial \Di`a*go"ni*al\, a.
   Diagonal; diametrical; hence; diametrically opposed. [Obs.]

         Sin can have no tenure by law at all, but is rather an
         eternal outlaw, and in hostility with law past all
         atonement; both diagonal contraries, as much allowing
         one another as day and night together in one
         hemisphere.                              --Milton.