The University of Chicago Library Hanna Holborn Gray Special Collections Research Centercopy of Lowthorp’s abridgment of the Philosophical Transactions of the Royal Society of London has lightly written numbers by many figures on the illustrated plates. In most cases, they are reversed as they are in the image above.
These numbers would have been written forward on the engraved printing plate, so they would have printed backwards. Some numbers, however, are forward reading now which means they were written backwards on the printing plate.
These illustrations reproduce dozens of figures per plate from the Transactions. John Lowthorp renumbered the illustrations in a different order, so these numbers were written by the engraver to guide the work for one of two reasons that I can think of.
I think that either they laid out the plate in advance or had a separate person to engrave the numbers. In the first case, the engraving team would have written a small number where the figure would go. Since the plates packed in dozens of figures, laying them out would have made sure to fit them all in together.
On the other hand, it might be that whoever cut the figure numbers did not cut the original images. Many of the geometric figures also use guide lines to achieve accuracy, so it might have been that two specialized engravers were involved. One engraver copied good geometric diagrams; one engraver cut the numbers onto the figures.
