title: The visby lenses.
abstract: OBJECTIVE
  In this study, 10 lens-shaped rock crystals, manufactured not later than the early Middle Ages, have been examined with respect to their image-forming qualities.
  
  
  METHODS
  The spherical aberration of the lenses served as a scale for comparison. Measurements have been taken with a specially designed light section method. Some of the examined lenses have a silver mounting and have been used as pendants, whereas others are unmounted and show no signs of use as jewelry.
  
  
  RESULTS
  The two largest unmounted lenses have very interesting surface curvatures, rendering a very small spherical aberration of the lens. The combination of top and bottom surfaces of the lens causes an imaging quality comparable to the modern aspheric lenses used, e.g., in today's projectors.
  
  
  CONCLUSIONS
  According to the results of this study, the knowledge and comprehension of optical design was much further developed in the Middle Ages than we assume today.
authors.affiliation: Aalen University of Applied Science, Germany.
authors.forename: O
authors.initials: O
authors.lastname: Schmidt
authors.forename: K H
authors.initials: KH
authors.lastname: Wilms
authors.forename: B
authors.initials: B
authors.lastname: Lingelbach
mesh_terms: D049691:History, Medieval; D007910:Lenses, Intraocular / Q000266:history / Q000592:standards; D055095:Optics and Photonics / Q000266:history; D011474:Prosthesis Design / Q000266:history / Q000592:standards; D013499:Surface Properties; D013548:Sweden