Reviews : On the anomalies of accommodation and refraction of the eye, with a preliminary essay on physiological dioptrics, by F.C. Donders ... translated from the author's manuscript by William Daniel Moore ... The New Sydenham Society, Vol. XXII., London, 1864.

Date:
[1865]
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Credit: Reviews : On the anomalies of accommodation and refraction of the eye, with a preliminary essay on physiological dioptrics, by F.C. Donders ... translated from the author's manuscript by William Daniel Moore ... The New Sydenham Society, Vol. XXII., London, 1864. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    another at y on the retina. The point i will he distinctly seen j and i and y are conj ugate foci. When the principal focus is behind the retina of an eye in a state of relaxation, Donders applies the term hypermetropia to the condition, and the eye is said to be hypermetropic. In this condition, no objects, whatever be their distance, can be distinctly seen, except by the intervention of a lens, or by the exercise of an accommodating power of which we shall come to speak presently. Fig. 3 represents a hypermetropic eye. Its principal focus lies in (I>, behind the retina. The parallel rays, represented by continuous lines, converge, after refraction, towards <p‘} but they are intercepted by the retina before they reach that point; and they form a circle of dispei’sion round the point j on the retina. Thus, a point, even remote from the eye, is indistinctly seen ; aud nearer points, still more indistinctly. If the rays, however, are made to converge (as might be done by means of a convex lens) towards the point i, which is the focus conjugate withy on the retina, these convergent rays, represented by the dotted lines, are made to converge still more, and to unite on the retina in the pointy. In employing the terms emmetropia, brachymetropia, and hyper- metropia, Donders has taken, as the measure of the proportion or disproportion existing between the refracting power of the eye and its antero-posterior diameter, the position and distance of the point which is the focus conjugate with the point where the optical axis, or, what comes to the same thing, the visual line,* cuts the retina. Thus, * The optical axis is a straight line drawn through the centres of the refracting surfaces, so as to cut them at right angles. Strictly speaking, no single straight line does so ; but as the axes of the different refracting surfaces in the human eye are nearly, though not exactly, coincident, we may regard them as having a common axis. The visual line is a straight line drawn from the fovea centralis in the yellow spot to the point on which the eye is directed. It very rarely coincides with the optical axis ; but, from the form of the retina, the points where the