Magnification of images was well understood by the time the first devices were constructed in the early 18th century. Basically, the image is placed at the focal point of a biconvex or plano-convex lens (a magnifying glass) allowing someone to view the magnified image at varying distances on the other side of the lens. Court and von Rorh (1935) suggested that the popularity of the early French models was because they allowed presbyopic purchasers to see the images clearly despite the French fashion against wearing glasses. But Chaldecott (1953) doubted this could have been the sole reason for the devices' enduring popularity, leaving the major factor to be the realistic appearance of the depicted images.