Carpenter's principles of human physiology . Myopic Eye. i u^uiici a given eye la liux- mal or ametropic, and whether the ametropia is myopic or hypermetropic,is that known as Schemers experiment (Fig. 264); in this, a card or thinpiece of metal perforated with two small holes, the distance between whichmust be less than the diameter of the pupil, is placed before the eye. If theeyes be n ormal, two circles of light will be seen, which overlap one anotherto a cert am extent and in this brighter part all objects appear single, becausethe rays are united upon the retina at c. If, however, the ax
Carpenter's principles of human physiology . Myopic Eye. i u^uiici a given eye la liux- mal or ametropic, and whether the ametropia is myopic or hypermetropic,is that known as Schemers experiment (Fig. 264); in this, a card or thinpiece of metal perforated with two small holes, the distance between whichmust be less than the diameter of the pupil, is placed before the eye. If theeyes be n ormal, two circles of light will be seen, which overlap one anotherto a cert am extent and in this brighter part all objects appear single, becausethe rays are united upon the retina at c. If, however, the axis of the eye betoo long, as in Myopia, and the retina be at p q, the rays will have crossed, anddouble images will be seen in the central part of the field of vision ; whilst if. SENSE OF VISION. 727 the axis of the eye be too short, as in Hypermetropia, the rays will not have united, and double images will also be seen. By placing a piece of red or green glass over one of the holes, as at g, it is easy to determine whether the ray passing through that hole has or has not crossed* The term presbyopia, as limited by Donders, simply expresses a deficient power of accommodation in the eye, resulting from increased density of the lens, or from defective power of the ciliary muscle, so that the near point recedes beyond a certain point, arbitrarily fixed by Donders for the sake of convenience at eight inches. The effects of Atropine of Duboisia upon the eye are very remarkable, not only in dilating the pupil to the utmost in the course of from twenty to twenty-five minutes, but in completely paralyzing the ciliary muscle, and with it the power of accommodation, so that the near point becomes gradually more and more distant, till at length it coincides with the far
Size: 1816px × 1376px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1880, booksubjectphysiology, bookyear1
