. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. s i a and j a proceed, is at a Less distancefrom the lens than amounts to the focal distance V of the Lens. Therays then acquire after refraction through the Lens, as be and // <?,a direction as if they had come from a point more distant from tin-lens. In the explanation of ordinary vision this last is, however, ofno importance, inasmuch as objects are always held far enough from * This poiDt is called the centre of the lens, and its position depends onlyon the curvatures of th
. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. s i a and j a proceed, is at a Less distancefrom the lens than amounts to the focal distance V of the Lens. Therays then acquire after refraction through the Lens, as be and // <?,a direction as if they had come from a point more distant from tin-lens. In the explanation of ordinary vision this last is, however, ofno importance, inasmuch as objects are always held far enough from * This poiDt is called the centre of the lens, and its position depends onlyon the curvatures of the two surfaces.—S. II. t Or focus conjugate to the point from which the rays come. Opticalwriters use the word focus, in general, for the point of union of rays; andthe focus of parallel rays is by them called the principal focus. When therays are not parallel, the points of divergence, and of union of the rays, arocalled conjugate foci—8. H. 8 PROOFS OF ACCOMMODATION. the eye to allow of the rays proceeding from them being brought, ifnot into union, at least into a converging direction. Fiar. 4. Fie. 3. In the normal eye, the retina is placed precisely at the focal dis-tance of the dioptric system. Parallel rays, derived from infinitelydistant objects, are therefore brought into union exactly in the objects are accurately perceived. From near objects, as we haveobserved, the rays proceed in a diverging direction, and their pointof union in the normal eye, consequently, lies behind the retina,and yet the organ is capable of perceiving near objects also accurately. ILLUSTRATIVE EXPERIMENTS. 9 It therefore has the further power of bringing divergent rays intounion on the retina. Now, this power of bringing at will rays ofdifferent directions into union on the retina is the power of accom-modation of the eye. We can easily convince ourselves that the normal eye possessessuch a power. That we are able clearly and accurately to distinguishobjects at different distance
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