Portraits Engraving Collection at Yale
Before photography engravings were made from portraits as a method of distributing images widely. This Yale Digital Collections concentrates on "scientists" if you consider alchemists, Jesuits, historians and dramatists in that class.
The link address is: http://cwmldl.med.yale.edu/gsdl/cgi-bin/library?site=localhost&a=p&p=about&c=portengr&ct=0&l=en&w=utf-8