c. 1920's vintage medical research laboratory optical microscope manufactured by the bausch & lomb optical co., rochester, ny. optical microscopes typically use refractive glass to focus light into the eye or another light detector. mirror-based optical microscopes operate in the same manner. typical magnification of a light microscope, assuming visible range light, is up to 1500x with a theoretical resolution limit of around 0.2 micrometers or 200 nanometers. this optical device along with several others were found in a shuttered michael reese hospital research laboratory. comprised of black enameled cast iron with nicely aged yellow brass dials, eyepiece, etc.