Page:Handbook of Ophthalmology (3rd edition).djvu/25

Rh proceeded from a point 48 inches on the other side. In order to see a distinct and single image of the point $$a$$, the eyes must have adjusted themselves to such diverging rays without changing the convergence of their visual axes. We find, then, in this case, with the visual axes converging upon a point 12 inches distant, a relative near point 6 inches and a relative far point 48 inches from the eye. Now, since the ordinary adjustment of accommodation is upon the point of intersection of the visual axes, the range of relative accommodation is divided into two parts,—the one within, the other beyond, that point of intersection. That part of the relative range of accommodation which lies between the fixation-point and the relative far point is called the negative part, because, under ordinary circumstances, it is already used in accommodating upon the binocular fixation-point. The other part, extending from the point of intersection of the visual axes to the relative near point, is called the positive part of the relative range of accommodation, because, with the given degree of accommodation, it is not yet brought into use. It represents the unemployed power of accommodation. The results of a complete series of experiments made by Donders upon this subject may be stated as follows:

(1) With parallel visual axes the emmetropic eye is adjusted upon its far point. Its accommodation is fully relaxed. There can be no further relaxation, but there can be tension of the accommodation. All emmetropes having a good range of accommodation can see distinctly at a distance through concave glasses. In such cases, in spite of the parallelism of the visual axes, the accommodation must be adjusted upon the negative focal points of the concave lenses. Youthful individuals with a normal range of accommodation have, with parallel visual axes, an attainable relative near point at about 12 inches from the eye. With parallel visual axes, the relative range of accommodation is wholly positive; that is, a stronger tension but no further relaxation is possible.

(2) With convergence for the binocular near point no further tension, but only a relaxation of the accommodation, is possible; that is, the relative range of accommodation is wholly negative.

(3) With a degree of convergence for intermediate points, say at from 8 to 24 inches, as in ordinary work, the positive part of