On defects of vision which are remediable by Optical appliances : a course of lectures delivered at the Royal College of Surgeons of England.

  • Carter, Robert Brudenell, 1828-1918.
Date:
1877
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Licence: Public Domain Mark

Credit: On defects of vision which are remediable by Optical appliances : a course of lectures delivered at the Royal College of Surgeons of England. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    lens of one dioptric would improve distant vision, and there would be manifest hypermetropia to this extent. When the accommodation was fully paralysed, a lens of four dioptrics would be required; and the three dioptrics of defect which had been previously latent, or concealed by the accommodation, would become apparent and serve to make up the total. The proportion which the latent bears to the manifest or to the total hypermetropia must evidently be constantly changing, as the power of accommodation diminishes with advancing life. According to Donders, we may often have as much as six dioptrics of hypermetropia latent in childhood. By the age of twenty, as much as half of this, and by the age of forty more than three- fourths of it, will have become manifest. By the age of seventy, the whole of the hypermetropia will be manifest; and even a higher grade may be found than that which was at first supposed to be the total. Donders regards this apparent higher total of old age as being partly made up of what he calls hypermetropia ctcquisita; but, as I have already set forth, I think there is much reason to regard it as the real total, part of which had been concealed, in earlier life, by the resistance of the ciliary muscle to any single application of atropia. From considerations which have been already urged, it is manifest that the effort of accommodation, which is required in order to overcome hypermetropia, will be most easily made in con- junction with an effort of convergence. We have seen, in Fig. 18, that the emmetropic eyes of the subject from whom the diagram was taken, with their visual lines parallel, could exert only three dioptrics of accommodation; but that, when the visual lines were convergent to thirteen inches (n° 21'), the same eyes could exert seven dioptrics of accommodation. In hypermetropia, the conditions, although not exactly parallel, are strictly analogous; and hence cases of hypermetropia were divided by Donders into three classes, with reference to the relation of the defect to the convergence function. He calls the. hypermetropia Absolute, when accommodation for parallel rays is unattainable, even with the strongest convergence of the visual lines; Relative^ when there is