. A text-book of physiology : for medical students and physicians . (5280 feet). If we designate the size of the image as xand substitute the known values for the other terms of the equation. DIOPTRICS OF THE EYE. 313 we have — = 5^), or x = mm., which is about the diam-eter of the fovea centralis. The retinal image of the object inthis case would be, in round numbers, about roinnnr °f the actualheight of the object. Accommodation of the Eye for Objects at Different Dis-tances.—The normal or, as it is sometimes named, the emmetropiceye, is arranged to focus parallel rays more or less acc
. A text-book of physiology : for medical students and physicians . (5280 feet). If we designate the size of the image as xand substitute the known values for the other terms of the equation. DIOPTRICS OF THE EYE. 313 we have — = 5^), or x = mm., which is about the diam-eter of the fovea centralis. The retinal image of the object inthis case would be, in round numbers, about roinnnr °f the actualheight of the object. Accommodation of the Eye for Objects at Different Dis-tances.—The normal or, as it is sometimes named, the emmetropiceye, is arranged to focus parallel rays more or less accurately uponthe retina. That is, the refractive media have such curvatures anddensities that parallel, or substantially parallel rays are brought toa focus upon the retinal surface. When objects are brought closerto the eye, however, the rays proceeding from them become moreand more divergent. If the eye remains unchanged the refractedrays cut the retina before coming to a focus—so that eachluminous point in the object, instead of forming a point upon the. Fig. 129.—Diagram explaining the change in the position of the image reflected from thpanterior surface of the crystalline lens.—{Williams, after Donders.) retina, forms a circle, known as a diffusion circle. As thisis true for each point of the object, the retinal image as a wholeis blurred. We know, however, that up to a certain pointat least this blurring does not occur when the object is broughtcloser to the eyes. The eye, in fact, accommodates itself to thenearer object so as to obtain a clear focus. In a photographiccamera this accommodation or focusing is effected by moving theground glass plate farther away as the object is brought closer to thelens. In the eye the same result is obtained by increasing the curva-ture and therefore the refractive power of the lens. That a changein the lens is the essential factor in accommodation for near objectsis demonstrated by a simple and conclusive experiment devised b
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