Simple and Compound Light Microscopes

Abstract

The limiting resolution d, the distance between two points barely resolved by the human eye, is called visual acuity. It varies directly with the distance D between the object and the eye. In Figure 3.1 the eye’s viewing angle V is purposely exaggerated, and the distance D from object to eye is not drawn to scale with respect to d, which separates the barely resolved points P and P. Figure 3.1 demonstrates that when distance D’ to the nearer points P’ and P’ is exactly half the distance D, then the barely resolved distance d’ between P’ and P’ is exactly half of d. This direct relationship persists as the object is brought closer and closer to the unaided eye, until the object is brought to a certain minimal distance, approximately 250 mm from the normal eye. At this distance of closest vision, the eye can resolve two points that are about 0.15 mm apart. These limits are set by the NA = 0.002 for the eye with its iris diaphragm wide open. At this setting the eye’s lens is as thick as physiologically possible.⁽¹⁾