THE ACTION OF LIGHT 



that a combination of flint and of crown glass will 

 overcome the difi&culty. In a later chapter we shall 

 explain the difference between these two kinds of 

 glass. In practice, a plano-concave lens of flint 

 glass is combined with a double convex lens of crown 

 glass and, if the nature of the glass is satisfactory, 

 as also the shapes of the lenses, there is full cor- 

 rection for chromatic aberration, and objects viewed 

 through such a lens will not appear with coloured 

 margins. 



There is one further trouble likely to occur in 

 such, or any lens. We write of the rays meeting 

 at a point. In our diagrams we represent the rays 

 by straight lines, really they are much more com- 

 plicated than they appear in a diagram. It is quite 

 easy to take a ruler and make our imaginary light 

 rays meet at a point, as a matter of fact, where 

 real lenses and real light rays are concerned, it is 

 very difficult, if not impossible, to make the latter 

 meet at a single point. One more diagram may 

 make the matter clear. 



A — 



Parallel rays A pass through our lens and, as we 

 know, they should all meet at a point P, the prin- 

 cipal focus of the lens; the majority do so, but 



48 



