An accessory lens which may be inserted into the light path above the analyser in a transmitted-light microscope. When determining vibration directions or interference figures using parallel or convergent polarized light, the Bertrand lens is inserted to bring the image of the interference figures into focus. Alternatively, if the Bertrand lens is absent, the eyepiece may be removed and the vibration directions or interference figures observed by looking down the microscope tube. The lens was first used in 1878 by E. Bertrand who adapted an original (1844) design by G. B. Amici.