If the incident rays are convergent, as in Pl. XII. fig. 15, the focus _o_ will be situated nearer the lens than the principal focus, _f_. If, on the other hand, they are divergent, as in Pl. XII. fig. 16, the focus _f_ will be situated further from the lens than the principal focus _o_. By concave lenses the incident rays are rendered divergent, as in Pl. XII. fig. 17, as if they emanated from a point _f_, situated on the same side of the lens as that upon which the rays are incident, and called the virtual focus. _Spherical aberration._--Although, as a general expression, we have stated that the rays of light meet at a focus on passing through a convex lens, this is not strictly correct. For, in ordinary convex lenses, the marginal rays are more refracted than the central ones, and meet at focal points nearer the lens than the latter, as shown in Pl. XII. fig. 18. This important defect is called spherical aberration, and arises from the lateral rays being incident upon more oblique portions of the curved surface of the lens than the central rays. Hence objects seen through such lenses appear misty and confused, the central and lateral parts of a flat object not being visible at the same time; and even when the marginal parts are visible, they appear distorted or deformed. Spherical aberration is greatest in the most convex lenses; and, in a plano-convex lens, it is least when parallel rays enter at or emerge from its convex surface. In certain lenses, the convex surface of which has the form of a parabola, a hyperbola, or an ellipse, the spherical aberration is absent; but it is impossible to grind microscopic lenses of these forms with absolute accuracy, so that the fact is of no practical value. The form of simple convex lens most free from aberration is that in which the curves of the two surfaces form parts of a sphere, the radii of the curves being as 1 to 6; the focal length being rather less than twice the length of the radius of the most convex surface. This form of lens comes very near to a plano-convex lens, which is consequently the best form for a simple lens.