Atlas and epitome of ophthalmoscopy and ophthalmoscopic diagnosis / by O. Haab.

  • Haab, O. (Otto), 1850-1931.
Date:
1910
Catalogue details

Licence: In copyright

Credit: Atlas and epitome of ophthalmoscopy and ophthalmoscopic diagnosis / by O. Haab. Source: Wellcome Collection.

    33/376 (page 29)
    to an ex teilt, let us say, of two diopters and finds the correcting concave lens to be one of 5 D, lie must add bis o\vrn hypermetropia, and the patient’s eye in that case has a myopia of 7 I). An emmetropic observer in this case would need a concave lens of 7 D, but the liyper- metrope needs one of only 5 I), because bis own correct- ing convex glass of 2 D neutralizes the effect of a con- cave glass of 2 D. He would also need - 7 if he looked through the ophthalmoscope witli bis own correcting lens of 2 D. But it is better to liave only one lens behind the ophthalmoscope, hence he will need only — 5 instead of — 7 to correct the myopia of the examined eye. Measurement of the Hypermetropic Eye. Having now considered cases where the examined eye is either emmetropic or myopic, we must consider the possibility of its being hypermetropic. Again, we must consider how the rays of light emerge froman hypermetropic eye when it is illuminated witli the ophthalmosco]ie. An hypermetropic eve without accom- modation is focussed neither for parallel nor for diver- gent, but for convcrgent ravs of light (cf. Fig. B); that is to say, only convcrgent rays of light are collected on the retina to form a distinct image. Parallel rays of lischt enterin<r the eve unite to form an inume behind the retina at the point /, either because the axis of the hypermetropic eye is too short or because its refractive System is too weak, as, for instance, when the lens is absent. In either case the refractive System of the hy- permetropic eye is insuffieicnt as compared witli its axis, and therefore requires a re-enforeing lens. The hypermetropic eye can, by exerting its accommodation, incrcasc its refractive power by increasing the refrac- tion of the lens, so as to bring the image for ward and on to the retina. It differs from the emmetro])ic eye in the fact that it has to accommodate even when looking into infinity, but this can be obviated by the use of a con- vex lens. As a rulc, the visual error in the hypenne-