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8 OPTICAL PROJECTION simple lenses answer every purpose, and are preferable because they stop less light; some being lost at every piece of glass by reflection from its surfaces, and by absorption in its substance. For, in the third place, it will be manifest that the brightness of a projection must depend upon utilising in our final image a sufficient number of luminous rays. If we regard the rays proceeding from an object as equally luminous in every direction, the number of rays collected must obviously depend solely upon the size of the lens, and its distance from the object when focally adjusted : the brightness of the image will, however, further depend upon the screen-distance. By a very simple and obvious law, the light falling on a lens must diminish as the square of its distance from the object; and that on the screen, in the inverse ratio of the surface of the lens and that of the superficies covered by the rays on the screen ; both depending, as we have already seen, upon the focus of the lens. Hence comes the utility of large lenses, which are highly advantageous for many physical experiments in projection, as we shall see. But it by no means follows that a large lens will give more luminous results in every case ; for the result depends upon collecting an adequate number of luminous rays, and it is by no means to be taken for granted that the rays emitted from an object are equally luminous in all directions : it may rather be the case that all which are luminous enough to afford us much help towards our final image, are confined to a very limited range. To understand this is the last essential point in the problem of projection. 5. Use of a Condenser.—We have already had a proof and example of this. When we first removed the lens and pricked a hole in the tinfoil, the margin of the image of the slide was not illuminated, and we had to draw back the light considerably from its usual position before we could make it so. The slide itself was amply illuminated, we know, for with the lens the image