title: A method of determining the refractive index of a prismatic lens.
abstract: OBJECTIVE
  A new method of measuring lens refractive index requiring immersion in solution and measuring lens power in air and in solution is extended. Prompted by a clinical need, the new method using lens power can be extended by applying it to prismatic power as well. This article provides a theoretical basis explaining why this can be done.
  
  
  METHODS
  The prismatic power of a prism is derived from first principles. Snell's Law and geometrical optics provide the framework for demonstrating the validity of the resulting formula.
  
  
  RESULTS
  The sameness in formula derived using lens power or prism is shown, both from a paraxial and non-paraxial optics perspective. The effect of varying lens material and amount of prism is considered.
  
  
  CONCLUSIONS
  The prismatic method described provides a useful alternative method of determining the refractive index of any lens. In some cases, it may be the only method available. Practitioners should consider when each method will provide optimal results.
authors.affiliation: jgb@netspace.net.au
authors.forename: John G
authors.initials: JG
authors.lastname: Buckley
mesh_terms: D005139:Eyeglasses / Q000592:standards*; D006801:Humans; D012029:Refraction, Ocular; D012031:Refractometry / Q000379:methods*; D015203:Reproducibility of Results