111. Chandelier Crystals and Prisms. When a ray of light passes through plane glass, like a window pane, it is shifted somewhat, but its direction does not change; that is, the emergent ray is parallel to the incident ray. But when a beam of light passes through a triangular glass prism, such as a chandelier crystal, its direction is greatly changed, and an object viewed through a prism is seen quite out of its true position. Whenever light passes through a prism, it is bent toward the base of the prism, or toward the thick portion of the prism, and emerges from the prism in quite a different direction from that in which it entered (Fig. 68). Hence, when an object is looked at through a prism, it is seen quite out of place. In Figure 68, the candle seems to be at _S_, while in reality it is at _A_. [Illustration: FIG. 68.--When looked at through the prism, _A_ seems to be at _S_.] 112. Lenses. If two prisms are arranged as in Figure 69, and two parallel rays of light fall upon the prisms, the beam _A_ will be bent downward toward the thickened portion of the prism, and the beam _B_ will be bent upward toward the thick portion of the prism, and after passing through the prism the two rays will intersect at some point _F_, called a focus. [Illustration: FIG. 69.--Rays of light are converged and focused at _F_.] If two prisms are arranged as in Figure 70, the ray _A_ will be refracted upward toward the thick end, and the ray _B_ will be refracted downward toward the thick end; the two rays, on emerging, will therefore be widely separated and will not intersect. [Illustration: FIG. 70.--Rays of light are diverged and do not come to any real focus.] Lenses are very similar to prisms; indeed, two prisms placed as in Figure 69, and rounded off, would make a very good convex lens. A lens is any transparent material, but usually glass, with one or both sides curved. The various types of lenses are shown in Figure 71. [Illustration: FIG. 71.--The different types of lenses.] The first three types focus parallel rays at some common point _F_, as in Figure 69. Such lenses are called convex or converging lenses. The last three types, called concave lenses, scatter parallel rays so that they do not come to a focus, but diverge widely after passage through the lens. 113. The Shape and Material of a Lens. The main or principal focus of a lens, that is, the point at which rays parallel to the base line _AB_ meet (Fig. 71), depends upon the shape of the lens. For example, a thick lens, such as _A_ (Fig. 72), focuses the rays very near to the lens; _B_, which is not so thick, focuses the rays at a greater distance from the lens; and _C_, which is a very thin lens, focuses the rays at a considerable distance from the lens. The distance of the principal focus from the lens is called the focal length of the lens, and from the diagrams we see that the more convex the lens, the shorter the focal length.