Page:Handbook of Ophthalmology (3rd edition).djvu/26

20 the relative range of accommodation is greater than the negative part; that is, with this degree of convergence only the smaller part of the whole available accommodation is employed.

Bonders has presented these results diagrammatically. The principle of this diagrammatic representation is, that the distance between two horizontal lines represents a definite result of accommodation, which is assumed as the unit of measure. As this unit, $$\frac{1}{24}$$ is chosen,—that is, a result of accommodation equivalent to the action of a convex lens of 24 inches focal distance. We begin now, in Fig. 6, with the lowest line, marked $$\infty$$, which represents the eyes

Fig. 6.

2A 2f 3 34 44 6 12 24 / / 1 k/ / / . D P^ — > P^ >^ ^ 7 ^f. ^ >^ s V / ^ x / / y y y Y / /' y y A ^ r 0° 11°2F 22°50^ 34°32^ 4G°38^ 59°20' 72°50^

as adjusted for parallel rays. The second horizontal line, marked $$24$$, represents an increase of accommodation by $$\frac{1}{24}$$, or the adjustment of the eyes for a distance of 24 inches. The third horizontal line, marked $$12$$, represents an increase of accommodation by twice $$\frac{1}{24}$$; the third line, marked $$8$$, by three times $$\frac{1}{24}$$; and so on. The diagonal line, $$KK'$$, represents the convergence of the visual axes. The numbers in the column on the left express in inches the distance of the object. The numbers below the diagram express the angle of convergence. The distance between the eyes is assumed to be 64 millimetres.

The position of the relative near point for every given angle