Eyestrain (commonly called asthenopia) / by Ernest Clarke.

Ernest Clarke

Date:: 1892

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

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$pupil diminished in size. Both the ciliary muscle and the circular fibres of the iris are supplied by fibres from the third nerve. Amplititde of Accommodation—Kt rest the eye is adapted for the most distant point it can see distinctly, viz., its punctmn remotum R, while the greatest possible contraction of the ciliary muscle adapts the eye to the nearest point it can see distinctly, viz., its punctum proximum P, which represents the greatest possible contraction. The force required to change the eye from R to P is called the amplitude of accommodation and is represented by the difference between the refraction of the eye at rest and the refraction when doing its utmost work. The equation is :— a = p — r where a equals the number of dioptres represented by the accommodation, p equals the number of dioptres represented by the eye when in a state of maximum refraction, i.e., when adapted for its nearest distinct point, and r equals the number of dioptres represented by the eye at rest, i.e., when adapted for its furthest distinct point. In Emmetropia as R is at infinity a = p-co rr:p-0 = p therefore the amplitude of accommodation is represented by the nearest distinct point, if this is 9 cms. off'a'= = II D —that is, the power of accommodation this person possesses is equal to a lens of eleven dioptres. In myopia r has a positive value. Take for example, a person whose furthest distinct point with the eye at rest is 33 cms. (that is a myope of 3 d), and suppose that his nearest distinct point is 7 cms., then a = p — r 100 IQlO ~ 1 33 = 14-3 = II D$

Eyestrain (commonly called asthenopia) / by Ernest Clarke.

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