The numbers along the top refer to the ages, and those at the side give the number of dioptres. The line staring at zero and terminating in the curve r r shows the static refraction, which remains unchanged until the fifty-fifth year, when it diminishes. At this age the emmetropic eye becomes hyperopic, the hyperopic eye more hyperopic and the myopic eye less myopic.
The curve p p shows the maximum refracting power and how it progressively diminishes as age advances.
Both curves meet at the ages of seventy-three, which marks the point when accommodation ceases. The number of dioptres included between the two curves on any given vertical line expresses the amplitude of accommodation at the age which the vertical line represents.
The change in the amplitude of accommodation is the same in all eyes, whether emmetropic or ametropic, though it will be seen that the position of the near point will vary with the condition of the static refraction. Form the diagram we obtain the following table giving the range of accommodation at different ages.
YEARS RANGE OF ACCOMMODATION.
10…………………………………… 14.D
15…………………………………… 12 D.
20……………………………. 10.D
25……………………………. 8.50 D
30……………………………. 7.
35……………………………. 5.50 D
40……………………………. 4.50 D
45……………………………. 3.50 D
50…………………………….. 2.50 D
55…………………………….. 1.75 D
60…………………………….. 1. D
65…………………………….. 0.75 D
70…………………………….. 0.25 d
75…………………………….. 0
The recession of the near point does not give rise to any disturbance until it passes beyond the distance at which the person ordinarily reads, writes or sews. When this occurs such occupations become difficult, on account of the small retinal images obtained. This distance differs with different people, therefore 22 cm., the point selected by Donders as the commencement of presbyopia, is arbitrary, though it is customary to state that such is the case. In order to see at 22 cm. a positive refracting power of 4.5 D. is necessary, and if the range of accommodation is less than that presbyopia is present. As the emmetropic eye at forty has just that amount, presbyopia commences in such an eye at that age. A hyperopic eye, however, would become presbyopia earlier, and a myopic eye late, depending upon the degree of the ametropia. Thus a hyperope of 1. D. would become presbyopic at thirty-five, because the amount of his hyperopia deducted from his range of accommodation, 5.50 D.-1.D., leaves him but 4.50 D or just sufficient to bring the near point to 22 cm. A myope of 1. D would not become presbyopic until forty-five, because his myopia added to his range would give him a positive refraction of 4.50 D.
Thus it will be seen that it is always necessary to take a patient’s static refraction into account before fitting him for near work.
If presbyopia is considered present when the near point recedes beyond 22 cm., it is obvious that it corrected by that convex glass which brings it back to that point, and the amount is measured by the glass which does so.
Most text-books give a table showing the amount of presbyopia present in an emmetropic eye at various ages to assist the student in determining the amount in a patient.
As this is only approximate, and as all people do not want to hold their work at 22 cm., the wisdom of it is very questionable. It is far better to individualize every case. Book- keepers, painters, violinists, carpenters, etc., work at a greater distance than 22 cm., and watch-makers and engravers at a shorter distance. The best course to pursue is to find the distance at which their work must be held, and fit them for that particular distance.
The theoretical glass having been determined in this manner, it is necessary, as Dr. Norton first pointed out some years ago in a paper read before the Homoeopathic Medical Society of the County of New York, to test the muscles at the near point with the glasses on, and if exophoria be present their strength reduced and if esophoria be present their strength increased. The method of ascertaining this is described in the chapter on the muscles.
Professor of Ophthalmology in the College of the New York Ophthalmic Hospital; Surgeon to the New York Ophthalmic Hospital. Visiting Oculist to the Laura Franklin Free Hospital for Children; Ex-President American Homoeopathic Ophthalmological, Otological and Laryngological Society. First Vice-President American Institute of Homoeopathy : President Homoeopathic Medical Society of the State of New York ; Editor Homoeopathic Eye. Ear and Throat Journal : Associate Editor. Department of Ophthalmology, North American Journal of Homoeopathy, etc.