This is made possible by the action of the crystalline lens and the ciliary muscle to which it is attached. 通过晶状体及其所连着的睫状肌的动作，可以做到这一点。
A transparent,biconvex body of the eye between the iris and the vitreous humor that focuses light rays entering through the pupil to form an image on the retina. 晶状体，水晶体眼球中一种透明的双面凸体，位于虹膜与玻璃体之间，它可使穿过瞳孔进入眼球的光线聚集在视网膜上来形成图像。
