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, 



