Page:Handbook of Ophthalmology (3rd edition).djvu/32

26 PRESBYOPIA. It is known that tlie lens becomes harder with age. From the investigations of F. J. v. Becker,* this hardening begins at a very- early age, affecting first the nucleus of the lens and spreading to the periphery. At -a later age this hardening may be plainly recognized by the stronger reflection of light observable upon focal illumination of the lens. It is fair to assume that with the change in the index of refraction there occurs also a change in the elasticity of the lens, so that it gradually loses the power to respond to the contraction of the ciliary muscle by increasing the convexity of its curved surfaces.

The gradual withdrawal of the near point causes at first no in- convenience. According to Donders, the average distance of the near point in the emmetropic eye at 20 years of age is 3f inches, at 35 years about 6 inches : and it is very seldom that there is occasion to use the eyes upon objects so near. When the near point recedes to 15 inches or more, inconvenience is felt in all oc- cupations which require distinct vision of near objects, such as reading, sewing, etc. If the objects are placed within the region of distinct vision, — that is, beyond the near point, — the retinal images become too small to admit of continuous vision ; upon bringing the objects within the near point the retinal images be- come larger, but at the same time are rendered indistinct by cir- cles of diffusion. This indistinctness of the retinal images is less than it otherwise would be from the fact that simultaneously with the limitation of accommodation the pupil becomes smaller and the circles of diffusion are correspondingly decreased. PresbyojDes seek, therefore, in every occupation the greatest possible amount of light; in the evening, for instance, when reading, they will hold the light between the book and their eyes in order by strong illumination to reduce as much as possible the size of the pupil.

The senile changes in the lens cause not only a withdrawal of the near point, but may affect the far point in the same way. The lens is known to consist of a great number of layers, whose in- dices of refraction increase from the periphery toward the centre, and it has been proved that by reason of this arrangement the resulting focal length is shorter than it would be if the entire lens possessed the refracting power of the nucleus. Now, we


 * Arch. f. Ophth., Bd. ix. 2, pag. 19.