The refraction of the human eye : a critical study of the statistics obtained by examinations of the refraction, especially among school children / by B. Alex. Randall.

  • Randall, Burton Alexander, 1858-1932.
Date:
[1885]
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Licence: Public Domain Mark

Credit: The refraction of the human eye : a critical study of the statistics obtained by examinations of the refraction, especially among school children / by B. Alex. Randall. 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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    human eye; in health, as far as we may exclude pathological conditions in youth and in age, if the data are to be found : to extract all tliat is ger- mane to our purpose. And when grave omissions or vexatious errors, generally typographical, have to be pointed out, it will be with full reali- zation from the writer's own experience, of how hard it is to avoid error in statistical reports, and how impossible for one man to comprehend in his investigation all those points that to another may seem most interest- ing or essential. And if, in passing, we cannot conceal a regret that some study, jmost valuable in some respects, gives us no information in others, it will be with no thought of adverse criticism—rather in testimony to the value of that which is given. To begin this study with a series of definitions, where the subject has been so much written about, may seem unnecessary, if not presumptuous. Yet, since loose employment of technical terms constitutes the principal ground of fallacy in statistics, the cardinal vice, which can make figures the most lying of witnesses, it is essential that some points be absolutely defined, and then the data of various observers be marshalled under these headings with the strictest accuracy which the details at hand will permit. Waiving technicalities as far as possible : the refraction of the eye is the relation of the situation of the percipient retina to the focus of the lens system of the passive eye, for rays of light coming from distant ob- jects. When, without any effort, the focus for such rays falls on the per- ceiving layer of the retina, the eye is emmetropic. When the eye is rela- tively too long to meet this condition, it is myopic; its retina is in position to receive the focus only of rays coming from some point not remote ; it is nearsighted. When relatively too short, the eye is hypermetropic; without accommodative eflfort, or its equivalent, the eye can focus no rays of light that occur in nature ; distant objects form undefined images upon the retina; near objects form images still less clear. Vision is the perception of external objects through the agency of the rays of light coming from them and impressing the retina. Its acuity depends upon the formation of a sharp image upon this membrane, and upon the capacity of it, and the visual centres behind it, to take cogni- zance of the image so found. Granted the latter conditions in full mea- sure, the acuity of vision depends upon the transparency of the ocular media and the refraction, primary or fictitious, of the eye. By fictitious refraction is meant every deviation from its primary condition above de- fined, whether produced by accommodative effort or by optical appliances, such as lenses. Conditions of imperfect retinal perception and of non- transparency of the media before it, may for our purposes be grouped under the heading amblyopia, and practically laid aside, and we have before us the subject of the visual acuity of the human eye as dependent upon its refraction, this last being divisible into real or primary, and ficti- tious or apparent, refraction. The real or primary refraction is the