A loupé is used to magnify a frame of negative or a transparency so you can examine detail like focus, depth of field or blur caused by camera movement. I have two in the studio: a Nikon 8x MC aspherical bought in Singapore in 1996 and a Schneider 10x loupé bought in 2007 (AUD $260). The Nikon is used for a quick "once over" on trannies, while the Schneider is used to critically assess images shot with manual focus PC lenses where the focus peg and depth of field edges can be very indistinct at low magnification. I started many years ago with a $8.00 4x plastic loupé that had all five aberrations of Seidel and seemingly 20 more again — awfrul to squint through! So invest in a good one if you want to really see what you've captured in negs and on trannies (especially).
The Bausch & Lomb above looks a beauty.