Glass is the substance most commonly used for refracting the rays of light in optical work, the glass being worked up into different forms according to the purpose for which it { 128} is intended. Solids formed in this way are termed _lenses_. A lens can be defined as a transparent medium which, owing to the curvature of its surfaces, is capable of converging or diverging the rays of light passed through it. According to its curvature it is either spherical, cylindrical, elliptical, or parabolic. The lenses used in optics are always exclusively spherical, the glass used in their construction being either crown glass, which is free from lead, or flint glass, which contains lead and is more refractive than crown glass. The refractive power of crown glass is from 1.534 to 1.525, and of flint glass from 1.625 to 1.590. Spherical surfaces in combination with each other or with plane surfaces give rise to six different forms of lenses, sections of which are given in Fig. 65.
All lenses can be divided into two classes, convex or converging, or concave or diverging. In the figure, _b_, _c_, _g_ are converging lenses, being thicker at the middle than at the borders, and _d_, _e_, _f_, which are thinner at the middle, being diverging lenses. The lenses _e_ and _g_ are also termed meniscus lenses, and _a_ represents a prism. The line XY is the axis or _normal_ of these lenses to which their plane surfaces are perpendicular.
Let us first of all notice the action of a ray of light when passed through a prism. The prism, Fig. 66, is represented by the triangle BBB, and the incident ray by the line TA. { 129} Where it enters the prism at A its direction is changed and it is bent or refracted towards the base of the prism, or towards the normal, this being always the case when light passes from a rare medium to a dense one, and where the light leaves the opposite face of the prism at D it is again refracted, but away from the normal in an opposite direction to the incident ray, since it is passing from a dense to a rare medium. The line DP is called the _emergent_ or refracted ray. If the eye is placed at T, and a bright object at P, the object is seen not at P, but at the point H, since the eye cannot follow the course taken by the refracted rays. In other words, objects viewed through a prism always appear deflected towards its summit.