Early Work In Photography
A Text-book For Beginners

54 Early Work in Photography. the images formed by the rays passing through the sides, and those through the centre of the hole. We need, therefore, some means of causing the images formed by the outer rays to fall on the same same spot as those formed by the rays passing through the centre. In other words, we want a means to bend slightly inward the outer set of images. I he bending of the waves of light is usually accomplished by re- flection or refraction, and in camera work the latter is found more convenient. When light passes from a medium of given density into one of different density, its rays are bent or refracted, and a simple example of this is found in the use of a prism. Suppose, therefore, we make our pin-hole much larger than before, allow the centre to be open for the central rays to pass through, and fill the upper and lower part with prisms, what will happen? The central rays of light will pass through the centre, as before, while those through the top will be bent (or re- fracted) downward and those through the bottom will be bent upward, so that the three images coincide on the ground glass We shall need another prism at the right-hand side and one at the left-hand side of the hole, to refract the rays from those directions, and then as four straight lines will not fill a circle, we shall need other prisms in corners between Fig. 14. those already placed. In fact, we need an infinite number of prisms, arranged round a common centre, and this is sim- ply a lens in a primary form (Fig. 14).