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)

    38/170 (page 26)
    LECTURE II. ACCOMMODATION AND CONVERGENCE. Mr. President, i^HE two remaining factors of sustained clear vision, the JL accommodation and the convergence, require to be considered not only separately, but also in their relations to one another. The first of these two factors, the accomodation, was defined in my former lecture to be the power of adjusting the eye to see with equal clearness at varying distances within its far-point ; and Mr. Hulke, when he held office as Arris and Gale Lecturer, fully described in this theatre the mechanism by which the function is performed. There is therefore no occasion for me to occupy ground which has already been so well covered; and I need only point out the necessity which there is for accomodation, and the nature of the change which produces it. Taking the emmetropic eye as a starting-point, with its capacity for uniting parallel rays upon its retina, it is obvious that rays coming from compar- atively near objects will no longer be parallel, but divergent; and that the degree of their divergence will constantly increase as the object from which they proceed comes nearer and nearer. In order to overcome this constantly increasing divergence of the rays, and still to obtain clear images from near or from steadily approaching objects, the eye must possess the power of increasing