Physiologic optics : dioptrics of the eye, functions of the retina, ocular movements and binocular vision / by M. Tscherning ; authorized translation from the original French edition., specially revised and enlarged by the author by Carl Weiland.

  • Marius Tscherning
Date:
1904
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Licence: In copyright

Credit: Physiologic optics : dioptrics of the eye, functions of the retina, ocular movements and binocular vision / by M. Tscherning ; authorized translation from the original French edition., specially revised and enlarged by the author by Carl Weiland. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

    45/392 (page 17)
    Methods of Measuring the Focal Distance of a Lens. —The method most frequently employed by oculists consists in looking at exterior objects through the lens, subjecting the latter to sHght dis- placements. We then notice that exterior objects are displaced in the same direction as the lens if the latter is concave, in the contrary direc- tion if it is convex. In other words, if the eye is in front of the middle of the lens the rays reach it without any deviation; but if the eye is placed before a peripheral part of the lens it receives rays deflected by reason of the prismatic effect of the glass, and this effect is greater in proportion as the part through which the eye looks approaches the periphery (fig. i6). — To determine the focal distance of a lens we find in the test case the glass which neutralizes it (i). But we must remember that the numeration of the glasses in the test case is frequently not very exact. — Lenses have the same curvature on both sides; we have therefore = ' ^'; the index of the lens is approximately n=i.S, which means that the focal distance and the radius are nearly the same length (-^ = ' ^' = -^). — It was customary for a long time to number lenses according to their radius of curvature; as the index is generally a little more than 1.5, it wjould follow that the strong lenses would have a focal distance somewhat less than the number they bear, but in the case of convex glasses the error would be nearly compensated for by the influence of the thickness of the glass. Later, numeration by dioptrics (2) was introduced; and to obviate the necessity of changing the moulds in which glasses are ground the manu- (1) We can also use with advantage the American spherometer, a little instrument with which we measure the radius of curvature and thus indirectly the refracting power of the glass. (2) [In 1872 Monoyer, of France, first proposed the term dioplrie. He says in the Annales d'Ocu- listique, Vol. 68, page 111: C'est le pouvoir dioptriquc de la Icntille d'un metre ou 100 centimHres rfe lon- gueur /oca!e qui doit servir d'unitt Cetle unitt nous I'appcUerons unitt viUrique ou ddcimale de r^raction ou simplementr-DIOPTRIE—gi Von veul Mens nous permettre ce niologisme d6riv6 coi\formtment aux usages sdentifiques. This term has been adopted all over the world and in English can have only one phllo- logically correct translation, that is dioptry. This correct form has been employed, instead of diopter, all through this work ] — W.