A microscope is often used by an authentication expert to date the kind of printing used to make both photographs and ink-and-printing-press prints. Modern reprints and counterfeits are often identified because the microscope shows the printing is too modern. An 1870 print couldn’t have been made with a printing technology invented in 1985.
For an example, the picture below what ‘elecrostatic’ printing looks like under the 100X zoom power of a microsope. Electrostatic printing is the modern technology used to make Xeroxes, photocopies and on standard home computer laser printers. Large numbers of reproductions of antique prints and photos have been made using this type of printing.
Under the microscope, the modern elecrostatic prints are easily identified by the unique and quirky pattern of the pigment (ink). In this printing process the lines are made up of many tiny dust-like grains of pigment that have been fused (melted) to the paper. When you handle ink jet cartridges you will notice the pigment/ink is dusty and dry, and can get all over the place if you are not careful. In the printing process not all the grains of pigment make it to the intended area before being fused, so the print is identified by the many stragglers outside the lines. It looks like it needs a dusting.
If an ‘1880 lithograph’ or ‘Renoir etching’ looks like this under the microscope, it is clear the print is a modern reprint.