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
Fig. 27.—Forms of Lenses.
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.
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
