In reference to optic conditions, we possess a clear and precise definition of all the refractive states of the eye which extends back even to the time of Kepler (1611).
Donders discussed the problem thoroughly. The definition being purely physical, it is clearly understood as such. Refractive conditions are divided into three types: the far-sighted, or hyperopia; the near-sighted, or myopia, and between these two the normal, or, as it is correctly named, ideal refraction or emmetropia. In the latter condition, the parallel rays of light unite exactly on the retina after they have been bent by the cornea and elastic lens, thus producing a clear cut image and, consequently, the ideally perfect vision. When these rays are focused behind the retina, the far-sighted condition is present, and when in front of the retina, the near-sighted condition.
In both the last mentioned conditions, an indistinct image is produced
POOS F, Roy D. THE MYOPIC PROBLEM. Arch Ophthalmol. 1929;2(1):66–75. doi:10.1001/archopht.1929.00810020071007
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