人的眼球内有一套主要由睫状肌和晶状体组成的“自动调焦设备”。当看近物时晶状体变厚，而看远物时晶状体变薄，如此使物体影像始终准确而清晰地落在视网膜上。遗憾的是，这套“设备”用上四五十年后就容易失灵。随着年龄的增长，睫状肌和晶状体的伸缩力量逐渐减弱，晶状体老 Inside the human eye has a set of ciliary muscles and lenses mainly composed of “auto-focus device.” The lens becomes thicker when looking at the near object, and the lens becomes thinner when looking at the near object, so that the subject’s image always falls accurately and clearly on the retina. Unfortunately, this “device” can easily fail after four or five decades. With age, the ciliary muscle and lens scaling power gradually weakened, the lens old