Page:Handbook of Ophthalmology (3rd edition).djvu/22

16 from an infinitely distant point falls upon the cornea in parallel rays. Every contraction of the muscle of accommodation causes the eye to be adjusted upon some nearer point. The nearest point upon which, with the full power of accommodation, the eye can be adjusted is called the near point.

Donders has shown that a distinction is to be made between an absolute and a binocular near point. The binocular near point is the nearest point upon which the accommodation, with simultaneous convergence of the visual axes,—that is, with binocular vision,—can be directed. The absolute near point shows the maximum power of accommodation. But this greatest possible action of accommodation by which the eye is adjusted for the absolute near point can be accomplished only with a relatively too strong convergence of the visual axes,—that is, with monocular fixation.

These relations existing between the convergence of the visual axes and the action of accommodation were first fully appreciated by Donders, and by him explained. For the emmetropic eye, he showed that in general the accommodation is adjusted for that point in which the visual axes intersect each other; and conversely, in ordinary use under normal conditions, the visual axes converge toward the point for which the eye as an optical instrument is adjusted. Nevertheless there does not exist, as was formerly thought, a constant relation between the convergence of the visual axes and the degree of accommodation. With one and the same degree of accommodation, there may be a greater or less convergence of the visual axes; and conversely, with an unchanged angle of convergence, the degree of accommodation can be either increased or diminished. We call this space through which the accommodation can be effected without change in the direction of the visual axes the relative range of accommodation. With an unchanged angle of convergence of the visual axes, the accommodation can adjust the eye for the relative near point, and relax it for the relative far point.

Donders has not only given the proof of the existence of relative accommodation, but has also accurately determined the position of the relative range of accommodation, and of the relative near and far points for every given convergence of the visual axes.

With the help of prisms we are able, with unchanged tension of accommodation, to alter the convergence of the visual axes.