FOCUS—OPTICAL CENTER.
The focus of a lens is the point where the refracted rays or their prolongation meet; if the rays themselves intersect after refraction the focus is real, and if their prolongations meet the focus is virtual. The line passing through the centers of curvature of the two surfaces of a lens is called the principal axis and contains a point known as the optical center, which has the property by virtue of which, if a ray passes through it, the ray will not be deviated. The optical center can always be found by drawing two radii parallel to each other, one from each center of the curvature of the surface until the radii intersect their respective surfaces, then draw a line joining these two points. The intersection of this last line with the principal axis will give the optical center.