Having learnt that light is bent from its course, it might be supposed that all objects looked at through plate glass should appear distorted; but it must be remembered that the sides of the glass being nearly parallel, an equal amount of refraction occurs in every direction--so that, unless the window is glazed with uneven wavy glass, the object, for all practical purposes, does not apparently change its position, being neither moved to the right or the left, or upward or downward. In order to bend the rays of light in the required direction, the glass must be cut into certain figures called prisms, plane glasses, spheres, and lenses, some of which are shown in the annexed cut. (Fig. 289.) [Illustration: Fig. 289.] [Illustration: Fig. 290. A B. A double convex lens. C is a ray of light, which falls perpendicularly on A B, and therefore passes on straight to F, the focus. D D. Rays falling at an angle on A B, refracted to focus, F.] It would be tedious to trace out, by a regular series of diagrams, the passage of light through the variety of combinations of lenses; and as the plane, convex, and concave surfaces have been examined with respect to their effect on the reflection of light, they may be referred to again with regard to their influence in refracting light. In the latter it will be found that convex and concave lenses have just the opposite properties of mirrors; thus, a convex lens receiving parallel rays will cause them to converge to a focus. (Fig. 290.) The case of _short-sighted_ persons arises from too great a convexity of the eye, which makes a very near focus; and that of old people is a flattening of the eye, by which the focus is thrown to a greater distance. The remedy for the latter is a convex spectacle-glass, whilst a concave lens is required for the former, to scatter the rays and prevent their coming to a point too soon. [Page 302] The action of a concave refracting surface is again the opposite to a concave reflecting surface--the former disperses the rays of light, whilst, the latter collects them. A concave lens, as might be expected, produces exactly the contrary effect on light to that of a concave mirror. (Fig. 291.) [Illustration: Fig. 291. A B. A double concave lens. C., is a ray of light which falls perpendicularly on A B, and passes through without any alteration of its course. D D. Rays falling at an angle on A B, are refracted and diverged.] These facts are well shown with the aid of the lantern and electric light. The rays of light are refracted in a visible manner when received on a concave or convex lens, provided a little smoke from paper is employed, as in the mirror experiments. (Fig. 292.) [Illustration: Fig. 292. A. The electric light. B. The lens.] Bearing these elementary truths in mind, it will not be difficult to follow out a complete set of illustrations explanatory of the construction and use of various popular optical contrivances. [Page 303] CHAPTER XXIII.