Eyesight in schools : a paper read before the Association of Medical Officers of Schools, on April 15th, 1885 / by R. Brudenell Carter.
- Robert Brudenell Carter
- Date:
- 1885
Licence: Public Domain Mark
Credit: Eyesight in schools : a paper read before the Association of Medical Officers of Schools, on April 15th, 1885 / by R. Brudenell Carter. 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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![Bonders to be emmetropic ; that is, to possess eyes which are in correct measure or proportion. As long as the object of vision is distant twenty feet or more, the divergence of the rays which proceed from it is not appreciable, and the image is still formed correctly ; but, as soon as the object comes within twenty feet, and in a constantly increasing degree as it approaches nearer, the divergence of the rays becomes ^toofgreat for the refraction of the eye to overcome, so that, when they reach the retina, they have not yet been united. If the back of the eye were transparent, the light would pass through it, and the image would be formed in a posterior plane ; but, the back of the eye being opaque, the light is intercepted, and forms on the retina either an image which is more or less blurred and in- complete, or an absolute dispersion circle, according to the degree in which the refraction falls short of the requirements of the case. An emmetrope with passive eyes would, therefore, have no clear vision of a near object, were it not for the exercise of accommodation, which is accomplished in the following manner :—The ci'ystalline lens is, in youth, a highly elastic body, which, in the passive state of the eye, or rather]of the ciliary muscle, is compressed from front to back through the intermediation of the zonule of Zinn, and is held in a flattened condition. When the ciliary muscle contracts, it brings forward the attachment of the zonule, relaxes the compression, and leaves the lens free to assume a shape of greater convexity, in which it exerts a greater degree of refracting power, and has its focus nearer than before. The performance of the function is guided, of course, by the resulting sensory impression, the ciliary muscle contracting more or less, according to the distance of the object, and always sufficiently to allow the precise amount of expansion of the lens which will afford a clear image. Hence the emmetrope, with a passive eye, has clear vision of all objects which are more than twenty feet away ; and, by the exercise of accommodation, obtains clear vision of all objects which are nearer ; with the single reservation that the power of accommodation is limited, and that there is a degree of approximation—a near point—within which the divergence of the rays becomes greater than the eye is able to overcome. It must be remembered, moreover, that the direction of the gaze](https://iiif.wellcomecollection.org/image/b21648360_0008.jp2/full/800%2C/0/default.jpg)
