object, say, a hundred yards away. B is a double convex lens, which has a focal length of twenty inches. We may suppose that it is a lens in a camera. An inverted image of the object is cast by the lens at C. If the eye were placed at C, it would distinguish nothing. But if withdrawn to D, the least distance of distinct vision,23 behind C, the image is seen clearly. That the image really is at C is proved by letting down the focussing screen, which at once catches it. Now, as the focus of the lens is twice d, the image will be twice as large as the object would appear if viewed directly without the lens. We may put this into a very simple formula:—
Magnification = focal length of lens d
In Fig. 124 we have interposed between the eye