Borrowing a technique from genetics,on June 20, 2006 S. Blair Hedges, professor at biology at Penn State, University Park, Pennsylvania published "A method for dating early books and prints using image analysis," Proc. R. Soc. Lond. A: Mathematical, Physical, and Engineering Sciences 462 (2006) 3555-3573, describing the "print clock" method for dating examples of printing, including books and copperplates, issued from hand-operated presses. A supplementary appendix was available from Hedges' website.