. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. he eye, lose the medullary sheath, and in their membranous expansion, asthe innermost layer of the retina, continue quite unchanged. The fibres ofthe optic nerve themselves are insensible to the stimulus of the vibrations oflight. Its entrance into the eye may be demonstrated as the blind spotof Mariotte ; and if we allow the little image of a distant flame to move to andfro on the papilla nervi optici, it is, as we can observe in ourselves, and inothers with the ophthalmoscope, i
. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. he eye, lose the medullary sheath, and in their membranous expansion, asthe innermost layer of the retina, continue quite unchanged. The fibres ofthe optic nerve themselves are insensible to the stimulus of the vibrations oflight. Its entrance into the eye may be demonstrated as the blind spotof Mariotte ; and if we allow the little image of a distant flame to move to andfro on the papilla nervi optici, it is, as we can observe in ourselves, and inothers with the ophthalmoscope, imperceptible ; it is not until the littleimage passes the boundary of the papilla, where the other layers of theretina are also present, that the feeble glimmering of light through thewhole eye gives way to a completely circumscribed image. As the membranous expansion is precisely similar to the fibres of thepapilla, we cannot expect to find in that expansion more sensibilityto the vibrations of light than exists in the nerve itself. And ifwe consider, that everywhere many layers of these transparent fibres 2. 4 CONDITIONS OF ACCURATE VISION. lie over one another, and that the same fibre runs from the ora serratato the papilla of the optic nerve, it is evident that rays, derived fromthe same point of an object pass, and necessarily irritate the fibres ofdifferent layers, and that the same fibre is struck by rays derived fromdifferent points of an object. Therefore, if the fibres were sensitive to theoscillations of light, the localized projection from the retina, or vision, asactually takes place, would be excluded.* 2. The layer of rods and bulbs, which is at least pretty generally con-sidered as the perceptive layer:—The supposition is, that the bulbs and rods,while they are traversed by the undulations of light, undergo a molecularchange, and that this change, whatever it may be, excites a secondary modi-fication in the fibres, which were incapable of directly experiencing
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