Eyestrain (commonly called asthenopia) / by Ernest Clarke.

  • Ernest Clarke
Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Eyestrain (commonly called asthenopia) / by Ernest Clarke. Source: Wellcome Collection.

    72/184 page 60
    It is very important to remember, that this table is only meant to show the average standard. In practice we often find considerable variation from it. This loss of accommodative power in the eye is due to diminished elasticity of the lens—in other words, the greatest convexity the lens can assume is at the age of 10; and every year afterwards, this convexity (which is the result of the fullest relaxation of the zone of Zinn) becomes less and less, till a time arrives, when it has lost all power of altering in shape. This loss of elasticity is accompanied by greater firm- ness of the lens, and in later years, by a loss of homogene- ousness and transparency both of the lens and vitreous, which is such a striking condition in youth. The lens reflects more light, and by oblique illumination often gives a false idea of cataract. At the age of 40 every individual has, more or less, about 4.5 D, range of accommodation (see table). From our formula a = p - r we get p = a+r, i.e. p=4.54-r. If the person is an emmetrope r = 00 =0 /. p = 4.5 ; now P. the punctum proximum is therefore p= L!H£ti5= 22cms. That is, an emmetrope at the age of 40 cannot read distinctly nearer than 22cms. or 8^ inches. This is the point at which Bonders has fixed presbyopia. At what age will a myope of 2 D. have his near point removed to 22cms. ? a = p - r = 4.5-2 = 2.5 By our table we see that, when the amplitude of accom- modation has diminished to 2.5, the person's age is 50. Hence a myope of 2 D. becomes presbyopic ten years later than an emmetrope. If the amount of myopia had been 4.5 D. a would have been o, and our table shows
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