*6 OPTICAL PROJECTION 



focus than the central rays. But when we use a simple lens 

 to form an image, there are other results from this. Eeferring 

 to fig. 15, it will be seen that the image mm of the arrow M M 

 is really brought to its focus upon a concave surface instead of 



FIG. 15. Spherical Aberration 



a plane ; or conversely, the diagram or picture m rn must be 

 drawn upon a concave surface to produce a flat image MM, an 

 expedient actually employed in early solar microscopes. 



FIG. 16. Distortions of Image 



The curved image maybe more or less ' flattened,' as it is 

 called, by adopting a meniscus form for the lens, as shown in 

 fig. 14 by the various achromatics B, c, E. But we are now 

 confronted with another result of aberration, in the form of the 



