"A loupe (pronounced 'loop') is a simple, small magnification device used to see small details more closely. Unlike a magnifying glass, a loupe does not have an attached handle, and its focusing lens(es) are contained in an opaque cylinder or cone, or fold into an enclosing housing that protects the lens when not in use. Loupes are also called hand lenses."
A printer's loupe is usually small---this one, when folded up, is about the size of a large postage stamp.
Opened, it holds the lens at the correct distance for focusing on a detail of a printed page. This built-in positioning eliminates having to fuss with and adjust the position of the lens while also trying to inspect the printed image.
Place the opened loupe on the printed area, and by then looking through the top of it...
...it is possible to see the dot pattern that makes up an offset reproduction (the usual technique by which most images are reproduced today), as well as other details. Being able to look at the printed area so closely helps with color correction, and with improving the overall reproduction.
When finished, fold it up and tuck it away for the next time.This loupe probably originally had a fitted leather case in which to store it--now lost. Loupes are still being made, and some of them are more complex than this one. But the modern loupes are plastic, and aren't always designed to fold up. I love the heavy solid feel of this little brass version, its collapsibility, and the fact that it did its job in Dad's studio for over 70 years.