Eyestrain (commonly called asthenopia) / by Ernest Clarke.

Ernest Clarke

Date:: 1892

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

66/184 page 54

$In Other words, 14 D would represent his amplitude of accommodation if he were emmetropic, but being myopic to the extent of 3 D we must subtract that, which leaves us II D to represent his amplitude. In hyperopia, as we shall see later, r is negative, therefore the equation is :— a = p - (- r) = p + r Thus, an eye hyperopic to the extent of 5 D,. having its near point at 25 cms. from the eye, has an amplitude of accommodation equal to a lens 9 D. To see 25 cms. off the eye requires an accommodation of 4 D (^°-), but it has already expended 5 D for distance, so that a = p - ( - r) = 4 - (- 5) = 4 + 5 = 9 D We thus see that to determine the amplitude or range of accommodation we must find R and P. R is represented by the refraction of the eye at rest. P we find as follows:— Take a tape, graduated on one side in centimetres, and on the other in corresponding dioptres. The zero-end of the tape is attached to the handle of a frame, into which may be introduced either a perforated diaphragm, or a paper with fine print upon it, or threads or hairs. This frame is brought towards the eye that is being examined (the other one being covered), until the objects begin to appear indistinct, we then read off on the tape the distance of P from the eye, and the corresponding dioptres (p) representing the maximum refractive power of the eye (Landolt). If from any cause, such as presbyopia or high hyper-$

Eyestrain (commonly called asthenopia) / by Ernest Clarke.

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