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

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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$modation which constitutes presbyopia, I mentioned his beUef .that the emmetropia of adult age often, or even generally, passes into hypermetropia as hfe advances. It is unquestionably often true that, after the age of fifty, distant vision is in some slight degree improved, or at all events is not impaired, by looking through a weak convex glass. The smaller curve of the diagram shows what is an ordinary course of things ; that is to say, that a convex quarter dioptric will be tolerated for distance at the age of sixty, half a dioptric at sixty-five, 075 at seventy, i*o at seventy- five, and I'50 at eighty. There can be no doubt about the facts ; but I venture to think there may be doubt about the right inter- pretation of them. I am disposed to regard such cases not as instances of acquired hypermetropia, but as instances of manifest hypermetropia which was previously latent, or entirely concealed by the accommodation. Bonders assumes that emmetropic eyes are not uncommon ; I almost doubt whether they exist. I use the term, in common parlance, to include all those very small degrees of hypermetropia which are not disturbing to vision, and which are perhaps too slight to be readily estimated or even identified; but, speaking with precision, I can hardly imagine an eye that is truly emmetropic. We talk about men who are six feet high, but the chances against any particular man being pre- cisely of that height are so great as to be almost incalculable. So they are, as it seems to me, against the focal distance and the axial length of any eye being precisely identical; and, of the two forms which ametropia may assume, hypermetropia is greatly the more common. When Dr. Cohn first investigated the refraction of children in the schools at Breslau, he found only 239 in 10,060, or 2-38 per cent, who were hypermetropic; but it has since been well estabhshed that the tests which he used were not fine enough for the discovery of the slighter grades of the condition ; and that, in children who are not myopic, a slight degree of hypermetropia is the rule. I have lately been told, by Dr. Hirschberg, of Berlin, of some investigations, the precise reports of which I have not yet been able to procure, but according to which the hypermetropic eye is the normal formation in children, and in all the lower animals$