Microscope 'selon Euler' by Chevalier, 1825-1826.
© Science Museum / Science & Society Picture Library
Compound microscope made by Vincent Chevalier (1770-1841) 'according to Euler' (Swis mathematician Leonhard Euler (1707-1783)). At its simplest, a compound microscope needs just two lenses; a small but powerful object lens, and a larger eyepiece lens. Light from the specimen is focused by the object lens so that it forms a magnified 'real' image. The eyepiece lens then enlarges this just like a magnifying glas, so that the observer's eye traces the light back to see a much bigger 'virtual' image. This example is designated 'selon Euler' because it incorporates an achromatic combination of lenses which are doublets constructed on principles published by Euler in 1774.