Multiple images are produced by a transparent parallel plate of glass. If the glass be silvered at the back, as it usually is in the microscope-mirror, the second image is brighter than the first, but as the angle of incidence increases the first image gains upon the second; and if the luminous object be a lamp or candle, a number of images, one behind the other, will be visible to an eye properly placed in front. This is due to the fact that the reflecting power of a surface of glass increases with the angle of incidence. [Illustration: Fig. 7.--Conjugate Foci of Curved Surfaces.] =Concave Surfaces.=--Rays of light proceeding from any given point in front of a concave spherical mirror, are reflected so as to meet in another point, and the line joining the two points passes through the centre of the sphere. The relation between them is or should be mutual, hence they are termed _conjugate foci_. By a _focus_ in general is meant a point in which a number of rays of light meet, and the rays which thus meet, taken collectively, are termed _a pencil_. Fig. 7 represents two pencils of rays whose foci, S s, are conjugate, so that, if either of them be regarded as an incident pencil, the other will be the corresponding reflected pencil. Each point, in fact, sends a pencil of rays which converge, after reflection, to the conjugate focus. _The principal focal distance is half the radius of curvature._ But it will not escape attention that concave mirrors have two reflecting surfaces, a front and a back. This, however, does not practically disturb its _virtual focus_, since the achromatic condenser when brought into use collects and concentrates the light received from the mirror upon an object for the purpose of rendering it more distinctly visible to the eye when viewing an object placed on the stage of the microscope. The images seen in a plane mirror are always virtual, and any spherical mirror, whether concave or convex, is nearly equivalent to a plane mirror when the distance of the object from its surface is small in comparison with the radius of curvature. Lenses. =Forms of Lenses.=--A lens is a portion of a refracting medium bounded by two surfaces which are portions of spheres, having a common axis, termed the _axis of the lens_. Lenses are distinguished by different names, according to the nature of their surfaces. [Illustration: Fig. 8.--Converging and Diverging Lenses.] Lenses with sharp edges (thicker at the centre) are _convergent_ or _positive_ lenses. Lenses with blunt edges (thinner at the centre) are _divergent_ or _negative_ lenses. The first group comprises:--(1) The bi-convex lens; (2) the plano-convex lens; (3) the convergent meniscus. The second group:--(4) The concave lens; (5) the plano-concave lens; (6) the divergent meniscus (Fig. 8).