Galilean telescopic system for the partially sighted. New application of the Fresnel lens.

A Galilean telescope consists of a convex objective lens separated from a concave ocular lens by the sum of their secondary focal lengths.The optical system is an afocal one, i.e. the incident and emergent light are parallel.The magnification (M) of this afocal system is obtained by dividing the dioptric power of the ocular (F2) by the dioptric power of the objective (FI), i.e.M = -F2/F1 = -f1/f2, where f, and f2 represent the focal lengths of the objective and eyepiece, respectively.Galilean telescopic systems aid the partially sighted by producing an apparent magnifi- cation of the conjugate retinal image.Typically, the telescopic systems used by the partially sighted are of two varieties: the spectacle telescope (in which the telescope is worn in the spectacle plane) (Appendix i), and the contact-spectacle-lens telescope (in which the con- tact lens serves as an eyepiece and the spectacle lens as the objective) (Appendix 2).Theoretically, there is also the contact-lens telescope, which should be mentioned in passing (Appendix 3).The usual types of telescopic spectacles, for examples the Bier, Feinbloom, Keeler, Stigmat, Zeiss, etc., are very large, heavy, cumbersome, and generally poor cos- metically.From an optical point of view, they induce a restricted visual field, varying between 70 and 200, depending upon the magnification.Consequently, the mobility of the patient is extremely limited, and many practitioners consider the reduced visual field intolerable, claiming that, if the patient was not legally blind before wearing the telescope, he would become legally blind, by definition, if he wore a telescope permitting so small a field.To overcome some of the above difficulties, the contact-spectacle-lens telescopic system was conceived and developed.This system usually consists of a lenticular ophthalmic lens (objective) and a concave contact lens (eye-piece) separated by the sum of their secondary focal lengths, equivalent in this case to the vertex distance, i.e. the distance between the spectacle plane and the corneal apex.Optically, the contact-spectacle-lens telescope offers the distinct advantage of a large field ofview, ranging from 430 to 500.We should point out that the magnifications attained with such an optical system, and the new Fresnel system, to be described below, offer no distinct advantage over those produced by systems mentioned previously, e.g. the Bier-Hamblin type.Cosmetically, however, the appearance of the con- tact-spectacle-lens telescope is still objectionable.In view ofthe above remarks, the following system is recommended as it alleviates the shortcomings of the various systems to which we