Eyestrain (commonly called asthenopia) / by Ernest Clarke.

Ernest Clarke

Date:: 1892

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

71/184 page 59

$ceases at an earlier age. In emmetropia p is seen to cross the zero line between the ages of 60 and 65 (fig. 3) (some accommodation is still left, but it is employed in cor- recting the acquired hyperopia ), and in hyperopia even earlier. The greater the degree of hyperopia, the earlier will p cross the zero line. In the case of a hyperope of 4D (fig. 4), this happens between the ages of 40 and 45. That is, a hyperope of 4 D when he reaches the age of 42, although he has some amplitude of accommodation, has to use it up entirely for distance ; on the other hand, all myopes of more than 3 D can make use of all the accommo- dative powers they have, for near work (fig. 5). These con- clusions were arrived at by Bonders, from a number of observations made and recorded,^ and fig. 3 is the result of the averages of these observations. By referring to fig. 3 we get the following table:— Every person ought to have at Age. Range of accommodation. 10 14 Dioptres 15 12 )« 20 10 25 8.5 » 30 7 » 35 5-5 40 4.5 » 45 3-5 » 50 ... 2.5 55 1.75 60 I » 65 75 JJ 70 25 i> 75 0 ' Accommodation and Refraction of the Eye, p. 209.$

Eyestrain (commonly called asthenopia) / by Ernest Clarke.

Licence and re-use

Contents