In fig. 27 we have the different forms of lenses shown in section. The first is the _double convex lens_, the second the _plano-convex_, the third and sixth the _concavo-convex_, the fourth the _double concave_, and the fifth the _plano-concave_. A _crossed lens_ is a double convex lens whose one side is more convex than the other. The third lens is also called _meniscus_. [Illustration: FIG. 27.—Forms of Lenses. ] The properties of the first, second, and third are similar; that is to say, they cause parallel rays of light passing through them to converge at a certain point, called their focus; while the three others have a divergent action on rays passing through them. By examining the path of the rays through these lenses, we shall find that the first three magnify objects seen through them, while the latter have the contrary effect. As in the case of the curved mirrors, the rays falling on the surface of a convex lens may be either parallel, divergent, or convergent. In the case of parallel rays, as depicted in the following figure, they are represented as meeting at a point beyond the lens, which is called the sidereal focus, or the focus for parallel rays. It is generally found by causing the image of the sun or of some distant object to be thrown by the lens upon a screen, or by knowing the curvature of the faces, and the refractive power of the glass. Every ray on striking the surface of the lens is refracted inwards, until it meets with its companions at the focus F, in accordance with the law of refraction, by which a ray of light passing from one transparent medium, such as air, to another which in this instance is glass, becomes refracted or bent in proportion to the relative density of the two mediæ. The nearer the ray passes to the edge of the lens, the more it is refracted, the angle of incidence being greater; the ray through the exact centre being uninfluenced by the form of the glass. Hence they all meet in a single point. Figs. 29 and 30 show the path of the rays when they are divergent and convergent. [Illustration: FIG. 28.—Path of a Ray through a Convex Lens. ] [Illustration: FIG. 29.—Path of divergent Rays through a Convex Lens. ]