. Elements of human physiology. Physiology. SPECIAL SENSES 553 muscular eftbrt in the eyes. There must tlien be some means l)y wliich tlie eye can l)e altered and arranged for focussing near objects. In a photographic camera the focus may be aUered either by changing the lens, putting in one of greater or less curvature, or by altering the distance of the screen from the lens. The last method is obviousl}' imprac- ticable in the rigid e3'eball, and we fnid that the act of focussing (or accommodating) for near objects is associated with a change in the curvature of the lens, which becomes more
. Elements of human physiology. Physiology. SPECIAL SENSES 553 muscular eftbrt in the eyes. There must tlien be some means l)y wliich tlie eye can l)e altered and arranged for focussing near objects. In a photographic camera the focus may be aUered either by changing the lens, putting in one of greater or less curvature, or by altering the distance of the screen from the lens. The last method is obviousl}' imprac- ticable in the rigid e3'eball, and we fnid that the act of focussing (or accommodating) for near objects is associated with a change in the curvature of the lens, which becomes more convex on its anterior surface. This may be easily shown by means of the phakoscope (Fig. 245). This is simply a box, blackened inside, with holes at a, b, c, and d. At (a) is the observer's eye; at (b) the observed eye. Across the middle of (d) a wire is Diagram of phakoscope. A candle is placed at (c). The observer at (a) then sees three reflections of the candle from the eye at (b) : a bright erect image from the anterior surface of the cornea; a larger but dimmer erect image from the anterior surface of the lens ; and a small very dim inverted image from the posterior surface of the lens. These images must be observed first when the eye at (b) is accommodated for a distant object, and then when it is accommodated for the wire stretched across the opening (d). It will be noticed that the change of accommodation from far to near objects is accom- panied by a change in the second image (that from the anterior surface of the lens), which becomes smaller. The change in this image is more easily seen if the candle be made to throw two images on the eye by interposing a double prism at (c). Then, as the lens becomes more convex to. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Starling, Ernest Hen
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