In the figures given below, and in the following page, the dotted lines represent perpendiculars to the surface at the point where the ray falls, and it is seen that the usual law of bending _towards_ the perpendicular on entering, and away from it on leaving the dense medium, is in each case correctly observed. [Illustration: Fig. 1.] [Illustration: Fig. 2.] Fig. 2, termed a prism, bends the ray permanently to one side; fig. 3, consisting of two prisms placed base to base, causes rays before parallel to meet in a point; and conversely, fig. 4, having prisms placed edge to edge, diverts them further asunder. [Illustration: Fig. 3.] [Illustration: Fig. 4.] _The various forms of Lenses._--The phenomena of the refraction of light are seen in the case of curved surfaces in the same manner as with those which are plane. Glasses ground of a curvilinear form are termed _Lenses_. The following are examples. [Illustration: Fig. 1.] [Illustration: Fig. 2.] [Illustration: Fig. 3.] Fig. 1 is a biconvex lens; fig. 2, a biconcave lens; and fig. 3, a _meniscus_ lens. As far as regards their refractive powers, such figures may be represented, nearly, by others bound by straight lines, and thus it becomes evident that a biconvex lens tends to condense rays of light to a point, and a biconcave to scatter them. A meniscus combines both actions, but the rays are eventually bent together, the convex curve of a meniscus lens being always greater than the concave. _The Foci of Lenses._--It has been shown that convex lenses tend to condense rays of light and bring them together to a point. This point is termed "the focus" of the Lens. The following laws as regards the focus may be laid down:-- That rays of light which are pursuing a parallel course at the time they enter the Lens are brought to a focus at a point nearer to the Lens than diverging rays. The rays proceeding from very distant objects are parallel; those from objects near at hand diverge. The sun's rays are always parallel, and the divergence of the others becomes greater as the distance from the Lens is less. The focus of a Lens for parallel rays is termed the "principal focus," and is not subject to variation; this is the point referred to when the _focal length_ of a Lens is spoken of. When the rays are not parallel, but diverge from a point, that point is associated with the focus, and the two are termed "conjugate foci." [Illustration] In the above diagram A is the principal focus, and B and C are conjugate foci. Any object placed at B has its focus at G, and conversely when placed at C it is in focus at B. Therefore, although the principal focus of a Lens (as determined by the degree of its convexity) is always the same, yet the focus for objects near at hand varies, being longer as they are brought closer to the Lens.