Page:Handbook of Ophthalmology (3rd edition).djvu/27

Rh of convergence is shown by the line $$p^1p^2p$$, and the position of the corresponding far point by the line $$rr^1$$. The points marked on these lines were found by direct experiment.

The diagram shows that the eye, whose relative accommodation it illustrates with parallel visual axes, has its relative near point at a distance of 11 inches: with an angle of convergence of $$22^\circ 50'$$ the relative range of accommodation lies between a distance of 12 inches and about 4.5 inches from the eye. The binocular near point lies at $$p^2$$, where the near-point line intersects the diagonal $$KK'$$. If the convergence of the visual axes increases still more, for instance, to $$46^\circ 38'$$, the line $$p^2p$$ remains below the diagonal $$KK'$$; that is, the tension of accommodation remains behind the point of convergence of the visual axes; if this point is 3 inches from the eye, the accommodation is adjusted upon a point 3.8 inches distant. The absolute near point $$p$$ lies somewhat nearer, at 3.7 inches. It can, however, be reached only with an angle of convergence of about $$70^\circ$$, when the visual axes will intersect at a distance of about 2 inches. At this maximal tension the range of accommodation ends; the lines $$p^2p^1$$, and $$rr^1$$ end here in one and the same point.

The changes which can be observed in the eye during accommodation are the following:

(1) The pupil contracts in accommodation for near objects; it dilates for distant vision.

(2) The margin of the iris and the centre of the anterior surface of the lens move forward in accommodating for near objects; simultaneously the peripheral part of the iris sinks back, as must necessarily be the case, since the volume of the aqueous humor remains unchanged.

(3) The anterior surface of the lens becomes more curved in near, and flattens again in distant, vision. This phenomenon, evidently the most important connected with accommodation, may be directly observed by studying the images reflected from the anterior and posterior surfaces of the lens. In accommodating for near objects the image reflected from the anterior surface of the lens becomes smaller, a proof that the surface becomes more curved; its position, too, is changed, which is explained by the moving forward of the reflecting surface.

(4) The image reflected from the posterior surface of the lens