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ON PROJECTION 7 will be seen that for any given distance of the lens from the slide or object, there can be but one distance from the screen which will properly unite all the ray-images in one. Hence, if we want to produce images under various conditions, or of various sizes, it becomes necessary to have several focussing lenses. Further consideration and experiment will establish the fact that, with a fixed object and a fixed screen, there are two positions in which the lens will pro- duce an image, the focal distances being the same in each case, only reversed. The lens may be near the object, and produce an enlarged image ; or nearer the screen, and produce a diminished image. We have the first case when a photo- graphic lens is used in a lantern: the second case when the same lens is used in a camera. In almost all cases it is the first kind of focussing which is used in projection. Secondly, it is plain that the distinctness of the projection will depend upon the perfection with which the lens bends in all the ray-images to precisely one spot. Unfortunately, it will be found that a simple lens, made of one piece of glass, does not perform its office perfectly. The different colours— be they evident, or only as existing in white light—are not refracted alike, the blue rays being bent in to a shorter focus than the red ones : this is called the' chromatic ' aberration. And the rays striking the edges of the lens are also bent in to rather a shorter focus than those passing through its centre: this is called its ' spherical' aberration. Also, the outer parts of the image tend to a shorter focus than the centre part: another form of spherical aberration. Where great distinct- ness of image is necessary, as in a lantern for exhibiting views or diagrams, we have to correct these faults by constructing compound lenses made of different kinds of glass, with or without air-spaces between. We may also use a diaphragm or limiting stop, cutting off the worst of the marginal rays, which is often done in lanterns. But in most experimental projections, where only broad features have to be made visible,