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4 OPTICAL PROJECTION With each we get another image, and it will be seen how they confuse each other; while if we stop all the others each one is distinct, and each of the last four is outside, on the screen, that from the central hole. It will now be understood instantly, that if we could bend in the rays which form the four outside images, so that they would fall exactly on the same spot as the central image, we should have again but one and a distinct image, but five times as bright as from one pinhole. 4. Use of the Focussing Lens.—To do this is the sole operation of the focussing lens, in a lantern or any other form of projecting apparatus. We can bend rays of light easily, by sending them at an angle through the surface of some other medium of greater (or less) density than the air ; and the greater the angle at which the ray strikes the denser medium, the more it is bent; the amount of ' refraction ' at lifferent angles being connected by a simple law which need not be discussed here. We only need to remem- ber, that on entering the denser medium the ray is bent in towards the perpendicular, and in leaving the denser me- dium away from it. If, then, we have a piece of glass with inclined faces, called a prism (fig. 2), to whose faces the dotted lines N.I and N E are perpen- diculars or * normals,' 1 the ray s i will be bent towards the normal, to the path i E, and on leaving the prism will be similarly bent away from E N to E R. Thus it is permanently bent in, or rofracted, towards the thick side of the prism. 1 All angles in optics are reckoned from the perpendiculars or normals, not from the surfaces themselves.