[Illustration: Fig. 14.--A Virtual Image formed by Concave Lens.]
In Fig. 14 A B is the object and _a b_ the image. Rays incident from A and B parallel to the principal axis will emerge as if they came from the principal focus F; hence, the points _a b_ are determined by the intersections of the dotted lines in the figure with the secondary axis, O A, O B. An eye on the other side of the lens sees the image _a b_, which is always virtual, erect and diminished.
In the construction of the microscope, either simple or compound, the curvature of the lenses employed is usually spherical. Convergent lenses, with spherical curvatures, have the defect of not bringing all the rays of light which pass through them to one and the same focus. Each circle of rays from the axis of the lens to its circumference has a different focus, as shown in Fig. 15. The rays _a a_, which pass through the lens near its circumference, are seen to be _more refracted_, or come to a focus at a shorter distance behind it than the rays _b b_, which pass through near its centre or axis, and are _less refracted_. The consequence of this defect of lenses with spherical curvatures, which is called _spherical aberration_, is that a well-defined image or picture is not formed by them, for when the object is focussed, for the circumferential rays, the picture projected to the eye is rendered indistinct by a halo or confusion produced by the central rays falling in a circle of dissipation, before they have come to a focus. On the other hand, when placed in the focus of the central rays, the picture formed by them is rendered indistinct by the halo produced by the circumferential rays, which have already come to a focus and crossed, and now fall in a state of divergence, forming a circle of dissipation. The grosser defects of spherical aberration are corrected by cutting off the passage of the rays _a a_, through the circumferences of the lens, by means of a stop diaphragm, so that the central rays, _b b_, only are concerned in the formation of the image. This defect is reduced to a minimum, by using the meniscus form of lens, which is the segment of an ellipsoid instead of a sphere.
[Illustration: Fig. 15.--Spherical Aberration of Lens.]